HVAC
RTU fan running but no cooling? Carrier and Trane compressor lockout checks
Separate cooling demand, controller permission, compressor command and proof before deciding whether to wait, correct an inhibit or repair a recurring trip.
Direct answer
A rooftop unit's supply fan can run while the compressor stays off because mechanical cooling is not requested, a control timer or inhibit is active, or a safety fault has stopped the circuit. Read the active cooling request, compressor command, proving status and diagnostic history together before resetting anything. Carrier ComfortLink, Carrier RTU Open, Trane Symbio 700 and Trane Symbio 800 use different fault names and restart rules, so match the unit and controller to the manual before deciding whether to wait, correct a control condition or repair a recurring trip.
On this page
Why can the RTU fan run while the compressor stays off?Which controller and manual match the rooftop unit?Is there a cooling call at the controller, or only a warm-space complaint?Which Carrier ComfortLink points and faults separate delay from lockout?Why can Carrier RTU Open show a command while cooling remains off?What do Trane proving faults and cooling-inhibited states mean?Which inhibit or lockout causes fit the evidence?What airside evidence rules out a false no-cooling diagnosis?What must be true before a restart, and when should retries stop?Worked example: Carrier low-suction logic at 30°F outdoor airWhat proves cooling is restored?What belongs in the repair scope and repeat-call program?Sources, method and citationOpen the related tool for the calculation, checklist, or packet.
Why can the RTU fan run while the compressor stays off?
Fan means the indoor supply fan delivering air from the rooftop unit, not a condenser fan rejecting heat outdoors. Separate five observations: cooling demand, permission to operate, compressor command, proof feedback and actual refrigeration effect.
Table 1 compares the selected OEM sequences in the manual crosswalk below. These observation classes are diagnostic categories, not controller alarm names.
| Supply fan observed | Cooling request at unit | Compressor command / proof | Supported interpretation | Next evidence |
|---|---|---|---|---|
| On | None | Off / off | Ventilation, satisfied setpoint or occupancy/control-source condition | Mode, effective setpoint, schedule, zone input |
| On | Cooling demand exists; DX stage not yet requested | Off / off | Economizer operation or unmet staging criteria | Economizer status and SAT; distinguish cooling demand from mechanical stage request |
| On | Mechanical stage requested | Off / off; timer active | Normal anti-recycle waiting | Remaining time and last stop, followed by command after expiration |
| On | Mechanical demand persists | Off / off; inhibit present | Operating condition or command makes circuit unavailable | Inhibit, limit, sensor, configuration |
| Continuous | May persist | Off; mitigation active | Equipped R-454B Precedent: protective refrigerant mitigation | RDS diagnostic and mitigation state |
| On | Requested | Off; FroStat Trip active | Selected Precedent evaporator protection has interrupted compression | Coil/airflow, diagnostic and recovery |
| On | Requested | On/unproven | Command/protection/proof disagreement | Diagnostic, diagram, safety/proof evidence |
| On | Requested | Off; latched circuit fault | Circuit lockout; motor failure unproven | History and correction before diagnostic reset |
| On | Requested | On/proven; motor unconfirmed | Auxiliary proof does not prove motor operation | Actual motor current/operation, equipment power and protection evidence |
| On | Requested | On/proven; motor runs, warm air | Leave the compressor-not-starting branch; verify refrigeration and air delivery | Refrigeration, air temperatures, airflow |
| On | None or commanded off | Proof remains on | Contactor/proving disagreement requires fault investigation | Physical state, feedback, history |
Record the affected circuit: one can be unavailable while another cools, and two compressors need not mean two independent circuits. Trane Symbio 700 cooling troubleshooting specifically cautions against confusing manifolded compressors with independent circuits.
Which controller and manual match the rooftop unit?
Identify the model and serial number, refrigerant, controller, firmware/application, installed options and local wiring-diagram number. Carrier University rooftop troubleshooting covers electromechanical, ComfortLink, PremierLink and RTU Open as distinct control systems; their sequences and diagnostic evidence differ.
| Installed controller/application | Verified document and edition | Read first | Boundary |
|---|---|---|---|
| Carrier 48/50HC 04-28, ComfortLink 3.X, R-410A | 48-50HC-07T, October 2021 | REQ.C, AVL.C, ACT.C, OK.MC, TG.A/TG.B; expanded alarm text | Do not transfer HC code meanings to all ComfortLink large rooftops |
| Carrier RTU Open v5 | 11-808-696-01, August 7, 2023 | Cooling preconditions, configured inputs, compressor output and safety alarm | Base unit's wiring and equipment protection remain separate |
| Trane Precedent with Symbio 700 | ACC-APG002G-EN, July 2026 | Alarms/status; compressor command and proving input; circuit, refrigerant and installed RDS | Cooling-only and heat-pump timers differ; R-410A and R-454B LPC recovery differ |
| Trane IntelliPak 20-150 tons, Symbio 800, TD7 | RT-SVP011G-EN, February 2023, with current diagnostic article Trane diagnostic reference | Exact diagnostic, target, severity, persistence and reset level | Match firmware and installed options to this edition's diagnostic rows |
| Trane Odyssey, Symbio 700 | ACC-APG001C-EN, March 2025 Odyssey C | Identify as a split system; select its own application guide | Do not import Odyssey Frostat/input assignments into rooftop instructions |
Read the local display as well as the BAS front end. The Trane Precedent application reference and Trane Symbio training resources identify application and access resources; Standard and Advanced Symbio 700 configurations have different BAS capabilities. Compare local status with the BAS update time; a stale value or a historical alarm cannot explain a present inhibit without current supporting evidence.
Other controllers need their own literature. Retain initial commissioning evidence in the RTU installation and startup record; use this guide to resolve the operating-state disagreement.
Is there a cooling call at the controller, or only a warm-space complaint?
A warm room is a complaint; an active request at the unit is control evidence. Compare the actual zone input with the effective cooling setpoint, occupancy and selected control source. Then read the mechanical stage request independently of a thermostat Y indication or a BAS graphic.
An economizer can satisfy cooling demand with outdoor air while compressors stay off. Confirm that mode, supply-air temperature and mechanical stage request agree. The economizer and demand-control ventilation guide explains normal operation; use economizer fault detection when command, position and temperature evidence disagree.
Which control conditions can withhold mechanical cooling?
Carrier's August 7, 2023 RTU Open v5 guide requires valid supply-air and space-temperature inputs, the indoor fan on, outdoor temperature above the configured cooling lockout where valid, no active heat mode and expiration of a five-minute intermode timeguard before mechanical cooling. RTU Open v5 guide p. 43.
For active-economizer integration, that same RTU Open v5 guide requires damper opening greater than 90% for at least 7.5 minutes, supply and outdoor temperatures greater than Minimum Cooling SAT plus 5°F, and space temperature greater than effective cooling setpoint plus 0.5°F before adding mechanical cooling.
ComfortLink demand limits cap available staging, while configured CA.LO/CB.LO values affect low-ambient circuit availability. Precedent Symbio 700 Cooling Lockout BAS disables DX cooling but permits economizer cooling, and Cooling Capacity Enable can limit DX stages. Record the active command and its intended building sequence using Carrier 48-50HC-07T pp. 21, 34-35 or Trane ACC-APG002G-EN p. 83. Do not change a VAV application to thermostat control just to obtain an output.
Which Carrier ComfortLink points and faults separate delay from lockout?
On Carrier 48/50HC 04-28 ComfortLink 3.X, REQ.C reports requested cooling stages, AVL.C available compressors, ACT.C actual cooling stages, OK.MC compressor permission and TG.A/TG.B remaining timeguard. ACT.C is controller status; confirm physical compressor operation separately. Carrier 48-50HC-07T pp. 20-21 makes these separate points: a request with no available compressor calls for an availability investigation before a motor diagnosis.
Expand T051/T055 before interpreting them. They identify A1/B1; the Safety Trip and Detected After Turnoff expansions describe different events. The safety-trip expansion stops the compressor and adds a circuit strike; the after-turnoff expansion commands all compressor relays off while cooling mode and the indoor fan remain on. Current-sensor-equipped and pressure-based detection arrangements also differ. Carrier 48-50HC-07T p. 42.
T092/T093 identify circuit A/B suction-pressure transducer failures that prevent the affected circuit from running. T133/T134 instead identify low-suction operating alerts evaluated through saturated suction temperature. Distinguish failed sensing from a valid low reading. Carrier 48-50HC-07T p. 43.
Carrier's October 2021 48-50HC-07T manual identifies A163 and A164 as circuit A and B shutdown alarms after three strikes and directs the technician to investigate the alarm that caused those strikes. Carrier 48-50HC-07T p. 44.
What does the ComfortLink strike counter do?
Carrier's October 2021 HC manual allows another circuit operating opportunity after a normal strike-related 15-minute shutdown. The strike count clears only after continuous problem-free operation for configured RST.C, whose default is five minutes. Carrier 48-50HC-07T p. 35.
The HC manual changes strike behavior in service test. Verify normal demand, staging and protection after service-test operation.
Why can Carrier RTU Open show a command while cooling remains off?
Carrier's August 2023 RTU Open v5 guide specifies a three-minute minimum on-time and a five-minute minimum off-time for each compressor output during normal cooling operation. A restart request inside that off interval is waiting, not by itself a lockout. RTU Open v5 guide p. 43.
The configured Compressor Safety input normally raises an alarm without interrupting cooling, heating or supply-fan outputs. For heat-pump heating, available electric heat replaces mechanical heat. Consequently, an RTU Open command can coexist with a base-unit safety interruption; the controller output and the equipment's physical protection circuit must both be recorded. RTU Open v5 guide p. 52.
Carrier's August 2023 RTU Open v5 guide, p. 52, also lets a configured Door Contact inhibit mechanical cooling and heating after a default 60-second delay while available economizer cooling continues. Identify that input before calling the condition a compressor failure. The BAS/DDC controls guide explains the distinction between supervisory command and actual equipment response.
What do Trane proving faults and cooling-inhibited states mean?
Cooling inhibited describes unavailable cooling; it is not one universal Trane alarm. Record the diagnostic, circuit, persistence and reset level. On Precedent Symbio 700, compressor proof monitors contactor auxiliary status, which establishes feedback to the controller, not compressor motor current or delivered capacity. Trane ACC-APG002G-EN p. 72.
Precedent Symbio 700: proving trip versus proving lockout
Trane's July 2026 Precedent guide disables the affected circuit for 15 minutes when compressor proof is lost after more than five seconds of operation. Its normal minimum runtimes are three minutes on and three minutes off for cooling-only units, or five minutes on and three minutes off for heat pumps; protection functions can interrupt minimum operation. Trane ACC-APG002G-EN pp. 72, 105-106.
Trane's July 2026 Precedent Symbio 700 guide requires a diagnostic reset after four consecutive compressor-proving trips in the same operating cycle, or when the proving input fails to close within five seconds of the compressor on command. Ending the compressor call clears the proving-trip counter; preserve the event sequence before interpreting a later zero count. Trane ACC-APG002G-EN p. 106.
Which failed path does compressor proof identify?
Trane's October 2026 Symbio 700 cooling troubleshooting article places the high-pressure cutout (HPC) and discharge-temperature limit (DTL) in the contactor control circuit. An open protection can therefore remove contactor power and produce lost proof. Establish the actual open device or connection before blaming the output board.
For HPC or DTL evidence, check condenser-coil condition, correct condenser-fan operation and the model's refrigerant pressure/temperature data. Fan staging changes with equipment and outdoor conditions; an idle second fan alone does not prove failure. If the contactor proves on but the compressor draws no current, investigate its power, switching and motor-protection path using the unit diagram. If current and refrigeration response confirm operation, continue with capacity and air-delivery diagnosis.
IntelliPak Symbio 800: record the target and reset level
Trane's February 2023 IntelliPak Symbio 800 programming guide classifies Compressor Failed to Start as a latching circuit diagnostic with a local reset level because the commanded compressor failed to prove on. This fixed-speed-compressor row specifies no startup timeout; do not import the Precedent timing. Trane RT-SVP011G-EN p. 97; Trane IntelliPak diagnostic reference.
IntelliPak's same guide declares Compressor Contactor Failure when proof fails to turn off for six continuous seconds after an off command. Circuit compressors are commanded off, but condenser fans and EXV control continue in case a compressor remains physically on.
Precedent FroStat and R-454B mitigation
The selected Precedent G manual treats FroStat Trip as a non-latching circuit diagnostic: compression stops while the supply fan continues if heating/cooling still requests it. Resolve coil frost, air delivery and the installed input condition. Precedent G defines the active FroStat input as closed; Trane's cooling web summary describes a normally closed switch. Resolve polarity from the installed unit diagram and application before testing the input. Trane ACC-APG002G-EN pp. 79, 105.
On equipped R-454B Precedent units, refrigerant concentration detection, sensor failure or detection-system communication loss can disable all compressors and run the supply fan continuously at or above its defined minimum speed. Read Refrigerant Mitigation Active with the initiating diagnostic: sensor failure alone does not establish a leak. Trane's July 2026 guide requires mitigation for at least five minutes after detection returns to normal, and an unresolved fault keeps mitigation active. Trane ACC-APG002G-EN pp. 80, 103.
Which inhibit or lockout causes fit the evidence?
Match both the event and its shutdown target. Table 3 covers selected causes, with circuit, DX-only and complete-unit effects kept separate.
| Selected application / state | Trigger or verified value | Controller response | Recovery prerequisite | Evidence to collect | Source |
|---|---|---|---|---|---|
| HC ComfortLink timeguard / low-ambient availability | TG.A/TG.B remaining time; actual CA.LO/CB.LO settings | Circuit unavailable until its conditions permit | Timer and ambient prerequisites satisfied | Timer, OAT and configured cutout | HC 07T p. 21 |
| HC T051/T055, Compressor Safety Trip expansion | A1/B1 detection depends on fitted current-sensing arrangement | Circuit strike and compressor stop | Correct safety/proof cause before recovery | Expanded text, sensing option, high-pressure switch (HPS), condensate overflow switch (COFS), protection and wiring | HC 07T p. 42 |
| HC T051/T055, Compressor Detected After Turnoff expansion | A1/B1 off command disagrees with running feedback | Compressor relays off; cooling mode/fan remain on | Correct switching/feedback cause; documented alarm reset | Exact expansion, fitted sensing, actual running state | HC 07T p. 42 |
| HC T092/T093, suction-pressure transducer failure | A/B transducer signal cannot be read correctly | Affected circuit cannot run | Correct sensor/supply/connection; reset | Signal source, connections, exact alarm | HC 07T p. 43 |
| HC T133/T134 low suction | Timed low SST; Table 4 | Circuit strike, except stated very-low-ambient exception | Correct airside, feed/charge or measurement cause | SST, OAT, economizer state, airflow and history | HC 07T p. 43 |
| HC A163/A164 | Three circuit strikes before qualifying recovery | Circuit lockout | Correct initiating cause; documented reset | Preceding alerts, timestamps, RST.C | HC 07T pp. 35, 44 |
| RTU Open Compressor Safety alarm | Configured safety input active | Alarm; see RTU Open output distinction | Resolve underlying equipment protection condition | Output status and base-unit wiring/safety evidence | RTU Open p. 52 |
| Precedent S700 Cooling Lockout BAS | BAS lockout command active | DX disabled; economizer available | Resolve erroneous BAS command or document intended limit | Local command, control source and building sequence | Precedent G p. 83 |
| Precedent S700 FroStat Trip | Installed FroStat diagnostic active in cooling | Circuit compression stops; requested supply fan continues | Coil/input condition corrected and diagnostic automatically clears | Airflow, coil frost, exact diagnostic and installed wiring | Precedent G pp. 79, 105 |
| Equipped R-454B Precedent S700 refrigerant mitigation | RDS concentration, sensor or communication diagnostic | Compressors disabled; protective supply-fan operation | Initiating condition resolved and prescribed mitigation completed | RDS diagnostic, sensor communication, Refrigerant Mitigation Active | Precedent G pp. 80, 103 |
| Precedent S700 proving trip | Running proof lost; see Trane subsection | Affected circuit off for 15 minutes | No proving lockout; valid request and corrected cause | Command, proof, contactor and safety chain status | Precedent G pp. 105-106 |
| Precedent S700 proving lockout | Startup proof failure or repeated proving trips; see Trane subsection | Affected circuit locked out | Correct cause; diagnostic reset | Event sequence; which compressor lost proof | Precedent G p. 106 |
| Precedent S700 LPC trip / lockout | LPC input trip; counter recovery depends on refrigerant | Circuit disabled three minutes; fourth qualifying event latches | Correct cause; apply refrigerant-specific counter recovery below; reset latched fault | Refrigerant, OAT, LPC input, runtime and event counter | Precedent G pp. 106-107 |
| Precedent S700 contactor failure | Proving input active 5 continuous seconds while command is off | Circuit de-energized and locked out | Resolve physical/feedback mismatch, then diagnostic reset | Command, proof and actual contactor/running state | Precedent G p. 106 |
| IntelliPak S800 Failed to Start | On command without proven-on input | Latching circuit inhibit, local reset level | Correct protection/proving cause and local diagnostic reset | Full diagnostic and circuit, protection module information | IntelliPak G p. 97; Trane diagnostic reference |
| IntelliPak S800 contactor failure | Off-command/proof disagreement; see Trane subsection | Circuit off command; condenser/EXV control continues | Resolve physical/feedback fault; use matching reset documentation | Command, proof, physical state | IntelliPak G p. 97 |
| Precedent S700 condensate overflow lockout | Third overflow trip within 72 hours | Latching unit shutdown, including supply fan in normal operation | Clear drain/pan/switch cause, then reset | Water level, drain condition, switch input, event times; reconcile any continuing fan | Precedent G p. 102; Trane cooling reference |
Trane's July 2026 Precedent Symbio 700 guide clears the low-pressure trip counter after three minutes of circuit operation without a low-pressure trip on R-410A units, compared with ten minutes on R-454B units. Trane ACC-APG002G-EN pp. 106-107.
The Precedent G sequence also distinguishes the LPC input before startup from a trip during operation: an open input while the compressor output is off does not itself inhibit startup. Ambient conditions modify the controller's automatic startup allowance. Read that sequence as implemented; changing or defeating a protective switch is not a diagnostic step.
Precedent condensate overflow lockout normally stops the supply fan too. A continuing fan requires reconciliation of event timing, fan identity and operating mode before this diagnosis fits.
Carrier HC low-suction thresholds
Table 4 applies to the R-410A HC ComfortLink 3.X application in Carrier's October 2021 manual. SST means saturated suction temperature and OAT means outdoor-air temperature. Read configured values before using factory defaults; these temperatures are not universal pressure-switch settings or R-454B thresholds.
| Quantity | Published factory value / condition | Interpretation | Source |
|---|---|---|---|
| SST.1 | 20°F; below threshold for 5 minutes | Level 1 detection | HC 07T pp. 43, 109-110 configuration tables |
| SST.2 | 15°F; below threshold for 4 minutes | Level 2 detection | HC 07T pp. 43, 109-110 |
| SST.3 | 10°F; below threshold for 3 minutes using economizer, 1.5 minutes without it | Level 3 detection | HC 07T pp. 43, 109-110 |
| Fast low-suction detection | Below SST.3 minus 5°F for 20 seconds, when OAT is above 40°F | Additional fast detection condition | HC 07T p. 43 |
| Cold-ambient adjustment | Below 40°F OAT, reduce the three thresholds 1°F per 2°F below 40°F | Read actual settings before applying adjustment | HC 07T p. 43 |
| Detection timer recovery | SST above SST.O for 1 minute; SST.O factory default 18°F | Resets low-suction detection timers; different from circuit strike reset | HC 07T p. 43 and configuration tables |
| Very low ambient exception | OAT below 10°F | Circuit shutdown without a strike for this alert | HC 07T p. 43 |
What airside evidence rules out a false no-cooling diagnosis?
Compressor-off operation can be correct while a comfort complaint remains unresolved. Verify the served zone, useful supply airflow and actual cooling mode together.
| Observation to establish | Record / compare | What it resolves | Next action / link | Evidence basis |
|---|---|---|---|---|
| Fan is physically moving useful supply air | Fan command, actual rotation/speed, measured airflow and equipment fan data | Running motor does not establish design flow | Follow duct-static-pressure and startup guides | HC 07T startup/fan sections; Trane cooling reference LPC discussion |
| Filter, belt and coil condition | Restriction, belt drive condition, frost, blower condition | Airside causes can produce low-suction alerts | Correct the observed cause before interpreting charge | HC 07T p. 43; Trane cooling reference |
| Active economizer cooling | Cooling mode, outside/mixed/supply air, stage request, damper status | Compressor-off can be intended free cooling | Economizer FDD guide if sequence and evidence disagree | HC 07T p. 20; RTU Open p. 43 |
| Representative temperature measurements | Matched probes, locations, simultaneous entering/leaving readings, humidity | Separates measurement/air-mixing effects from equipment output | Delta-T diagnostics guide | Anvilfield measurement method; equipment performance targets apply |
| Cooling reaches the correct space | Served-zone identity, zone temperature trend, schedule/setpoint | Unit cooling and occupied-space comfort are separate findings | Check delivery/control branch | HC 07T control sequence; RTU Open p. 43 |
| Compressor genuinely running | Command, proof, measured current and refrigeration response | Moves job out of compressor-not-starting branch | Charge/airflow diagnosis with model chart | Precedent G proof section; Trane cooling reference |
Use duct static pressure and equipment fan data to resolve airflow. Compare entering mixed air with leaving air; return-air temperature alone is not necessarily the evaporator-entering condition when outdoor air is mixed in.
Record probe locations, simultaneous readings and humidity using the temperature-split diagnostic guide. After actual compressor operation and airside conditions are established, use superheat and subcooling diagnosis with the unit's charging chart. Temperature split, pressure and airflow targets come from equipment data at the measured conditions.
What must be true before a restart, and when should retries stop?
Save diagnostics and history, identify the initiating cause and document correction before recovery. A reset alone establishes no repair. Carrier's October 2021 HC manual, p. 2, requires power isolation with locks and tags before service and warns of multiple power switches. An off command does not establish isolation; electrical measurements follow the OEM servicing precautions.
| Found state | Required evidence before action | Permitted disposition | What blocks closeout | Source |
|---|---|---|---|---|
| Normal minimum-off timer | Demand persists, no active blocking fault, correct timing | Let normal sequence finish; observe command and proof | Timer complete but no command or unsuccessful proof | RTU Open p. 43; Precedent G minimum runtime |
| Configured inhibit / missing precondition | Identify exact input, setting or mode and why it is active | Correct the erroneous condition or document intended operating state | Inhibit merely removed without identifying its cause | HC 07T p. 21; RTU Open p. 43 |
| First auto-recovering trip | Capture initiating fault and corrective evidence | Observe the OEM recovery sequence after correction | Same safety event returns | HC 07T p. 35; Precedent G pp. 105-107 |
| Precedent FroStat protection | Coil/airflow cause resolved; exact diagnostic/input recovered | Observe automatic recovery under normal demand | Coil re-freezes or the diagnostic remains active | Precedent G pp. 79, 105 |
| Equipped R-454B Precedent refrigerant mitigation | Initiating leak/sensor/communication condition resolved; required post-normal interval completed | Allow the controller to leave mitigation through its normal sequence | Active mitigation, unresolved sensor fault or renewed alarm | Precedent G pp. 80, 103 |
| Latched circuit fault | Cause corrected, history retained, model-specific reset method known | Perform the documented diagnostic reset and verify normal operation | Reset alone is the repair record | HC 07T p. 44; Precedent G p. 106; IntelliPak G p. 97 |
| Proof says running after off command | Establish actual compressor/contactor state and correct feedback or switching fault | Resolve fault before restart | Continued unintended operation or unexplained proof | Precedent G p. 106; IntelliPak G p. 97 |
| Condensate lockout | Drain/pan/switch fault corrected; event history retained | Reset per controller and verify normal draining | Water level or repeated overflow remains | Precedent G pp. 101-102; IntelliPak G pp. 97-98 |
| Compressor runs but output is unverified | Normal mode, airside and refrigeration measurements at documented conditions | Continue verification | Warm delivery, renewed trip or inadequate capacity evidence | Trane cooling reference; Anvilfield closeout synthesis |
Which reset is documented after correction?
Carrier HC 3.X separates Reset All Current Alarms, R.CURR, from Reset Alarm History, R.HIST. Preserve history and use the current-alarm action appropriate to the corrected condition. Deleting history removes evidence without proving a repair. Carrier 48-50HC-07T p. 42.
For the selected Precedent Symbio 700, Service > Diagnostics > Reset Diagnostic is the documented local-interface path and resets all diagnostics. Capture the initiating faults before using it. IntelliPak Symbio 800 instead requires attention to each diagnostic's local/remote reset level. Trane ACC-APG002G-EN p. 99; Trane RT-SVP011G-EN p. 97.
When the same safety event returns, resume diagnosis instead of repeatedly resetting. Do not use safety jumpers, forced compressor outputs, canceled timeguards or altered protective setpoints to obtain cooling.
Before replacing a Symbio 700 board, establish power, command/proof, safety-circuit and connection evidence; verify the equipment configuration and applicable firmware against the service alert. Trane Symbio 700 board-return service alert reports boards returned with no fault found. Use the installed application's procedures; Voyager connector instructions do not transfer to Precedent.
Worked example: Carrier low-suction logic at 30°F outdoor air
At 30°F outdoor air, Anvilfield estimates adjusted Carrier HC thresholds of 15°F, 10°F and 5°F. A constant 9°F SST reaches level 2 first, stopping the circuit and adding a strike.
Subtract outdoor temperature from the adjustment reference, then divide the difference by two to obtain the threshold reduction. Subtract that reduction from each configured threshold.
| Input / operation | Value | Provenance / consequence |
|---|---|---|
| Application | Carrier 48/50HC 04-28, ComfortLink 3.X, R-410A | HC 07T |
| Configured threshold inputs | SST.1 20°F; SST.2 15°F; SST.3 10°F | Published factory defaults, HC 07T configuration tables |
| Adjustment reference | 40°F OAT | HC 07T p. 43 |
| Outdoor-air calculation input | 30°F | Outdoor-air input |
| Threshold reduction | (40 − 30) ÷ 2 = 5°F | Anvilfield calculation using Carrier's adjustment |
| Adjusted level 1 | 20 − 5 = 15°F; 5-minute duration | Anvilfield estimate; OEM duration |
| Adjusted level 2 | 15 − 5 = 10°F; 4-minute duration | Anvilfield estimate; OEM duration |
| Adjusted level 3 | 10 − 5 = 5°F; 3 minutes with economizer, 1.5 without | Anvilfield estimate; OEM durations |
| SST comparison input | 9°F held continuously from initially clear timers | SST input |
| First applicable timed condition | 9°F < 10°F for 4 minutes | Level 2 reaches its duration first |
| Fast-path condition | OAT above 40°F required | Does not apply at the selected 30°F condition |
| Strike exception | OAT below 10°F required | Does not apply at 30°F; normal strike logic applies |
| Sensitivity: SST input 10°F at the same OAT/defaults | Equals adjusted level 2; below adjusted level 1 | Only level 1 of these three conditions qualifies; 5-minute duration |
| Sensitivity: SST input 4°F at the same OAT/defaults | Below all three adjusted levels | Level 3 qualifies first: 3 minutes using economizer, 1.5 minutes without |
Method: Anvilfield estimates use Carrier's October 2021 HC defaults and ambient-adjustment rule, with the compressor operating, initially clear timers and constant SST. Strict below-threshold comparisons determine the first qualifying duration.
What proves cooling is restored?
Restored cooling requires normal demand, available and proven compressor operation, no renewed blocking diagnostic and measured cooling delivery. Match actual staging to requested staging; record any remaining unavailable circuit.
Return the equipment to ordinary control mode and remove temporary service overrides through the applicable procedure. Record verification start, finish and elapsed operation. Counter recovery does not establish a universal stabilization period.
| Record field | Before correction: enter this evidence | After correction: enter this evidence | Acceptance / disposition rule |
|---|---|---|---|
| Asset and application | Model, serial, refrigerant, controller, firmware, options | Same identity; changed component/configuration if any | Trace every result to the same unit and manual |
| Demand | Zone input, effective setpoint, occupancy, Y/BAS source, requested stages | Same points at closeout | Explain why cooling is requested or legitimately absent |
| Availability | Inhibit, timer, available stages, active diagnostic | Cleared cause and current availability | No unexplained blocking state |
| Protective operating state | FroStat, RDS/mitigation or unit-shutdown status where applicable | Recovered state and initiating-cause evidence | Protection recovered through the prescribed normal sequence |
| Command and proof | Output state, auxiliary/current proof, actual running observation | Command agrees with proof and physical operation | Investigate mismatch rather than mark pass |
| Protection history | Exact alarm text, circuit, time, trip count, event order | Correction, reset type/time and subsequent history | No renewed initiating trip in recorded observation |
| Airside | OAT, return/mixed/supply air, humidity, airflow/static and probe locations | Equivalent readings with load and fan mode documented | Compare equipment/design targets at actual conditions |
| Refrigeration | Running status; model-specified readings when operation permits | Required running pressure/temperature/current readings | Use unit-specific charging/performance data |
| Operating duration | Last start/stop, remaining timer, elapsed healthy run | Start/end of normal-operation verification | Distinguish minimum-off, trip-retry and counter-reset durations |
| Disposition | Failed branch and intended repair | Restored, partial capacity, or unresolved; follow-up and owner handoff | Document remaining unavailable circuits and limitations of current load test |
Field checklist
- Confirm the correct unit, served zone and supply fan.
- Record model, serial, refrigerant, controller, firmware, options and manual edition.
- Save active diagnostics, history, circuit identity and timestamps before reset.
- Read control source, occupancy, setpoint and cooling request; compare available stages, timers and inhibits.
- Compare compressor command, proof and actual operation with the applicable fault row.
- On equipped R-454B units, record detection and mitigation status, distinguishing concentration, sensor and communication faults.
- Establish the indicated airside, drain, power, connection, sensing or protection cause using equipment instructions.
- Record correction and restart prerequisites; preserve protective functions and use the documented reset.
- Verify normal control, staging and cooling delivery; record elapsed operation and subsequent diagnostics.
- Retain before/after readings, remaining deficiencies and follow-up with the asset history.
What belongs in the repair scope and repeat-call program?
Price the diagnosed work: access, history retrieval, control/proof testing, correction and verification under normal operation. Separate a proven contactor or sensor failure, wiring repair, airside or drain correction, controller replacement and refrigeration repair. A lockout is an operating state, not a repair-price category.
Retain the affected circuit and before/after evidence in the maintenance-agreement record. Identify covered maintenance, separately authorized corrective work and after-hours service in the scope.
For buildings with several RTUs, review repeat calls by asset, exact diagnostic and operating conditions, then update the preventive-maintenance program around the demonstrated cause. OEM trip windows control diagnostics; they do not prescribe the visit frequency. Alongside a multi-unit RTU service list, PorchRocket Ground Control offers owner information and decision-maker contacts as add-ons for account outreach.
Sources, method and citation
- Carrier HC 48-50HC-07T, October 2021. 48/50HC 04-28 ComfortLink 3.X controls manual. Cooling pp. 20-21; strikes p. 35; alerts pp. 42-44; defaults pp. 109-110.
- Carrier RTU Open v5, August 2023. Installation and Start-up Guide 11-808-696-01, August 7. Cooling p. 43; inputs p. 52.
- Trane Precedent ACC-APG002G-EN, July 2026. Packaged rooftop application guide. Runtime/proof p. 72; protection pp. 79-80; BAS p. 83; diagnostics pp. 99-107.
- Trane IntelliPak RT-SVP011G-EN, February 2023. 20-150-ton TD7 programming guide. Compressor diagnostics p. 97; selected edition.
- Trane IntelliPak diagnostic reference. OEM diagnostic targets, persistence and reset levels; compared with the selected programming guide.
- Trane Symbio 700 cooling troubleshooting. Precedent/Odyssey/Foundation cooling troubleshooting, October 21, 2026. Airflow, circuit layout and charging.
- Trane Symbio 700 board-return service alert. Board Failures (No Fault Found), October 30, 2026. Diagnosis before replacement.
- Trane Precedent application reference. Application selection and Standard/Advanced BAS distinctions; updated November 4, 2026.
- Carrier University rooftop troubleshooting. Undated single-zone RTU course; control-family distinctions.
- Trane Symbio training resources. Undated controller access, application and integration resources.
- Trane Odyssey ACC-APG001C-EN, March 2025. Split-system boundary reference; separate from Precedent rooftops.
By Anvilfield. Sources checked for the November 5, 2026 review. Update log: Clarified proof versus motor operation, added protection and power-path checks, strengthened isolation prerequisites, and rechecked the selected Carrier and Trane editions and fault/restart rules.
How to cite this page: Anvilfield, RTU fan running but no cooling? Carrier and Trane compressor lockout checks, 2026, https://anvilfield.com/field-guides/hvac/rtu-fan-running-compressor-lockout-carrier-trane/
FAQ
Why is my rooftop unit fan running but the compressor not starting?
The supply fan can serve ventilation while cooling is absent, delayed, inhibited or locked out. Use the observation table to compare request, availability, command and proof.
What is the Carrier rooftop compressor lockout fault code?
On Carrier HC ComfortLink 3.X, A163/A164 identify circuit A/B shutdown. Investigate the preceding strike-producing alert using the expanded-code explanation. Other Carrier families need their own code definitions.
What does a Trane rooftop cooling inhibited alarm mean?
Cooling is unavailable under present conditions. The exact Symbio diagnostic, circuit and persistence distinguish a command limit, protective interruption and latching fault. Use the Trane controller section.
How do I distinguish no control call from safety lockout?
Read unit demand first. No request points toward schedule, input, setpoint and control source; a request with a blocking diagnostic points toward that fault. A thermostat screen cannot resolve this.
How long should I wait for the compressor to restart?
For normal cooling, Carrier RTU Open v5 (August 2023) holds each output off for five minutes; Precedent Symbio 700 G (July 2026) uses three minutes. Read the remaining timer from the last stop. Fault retry and mitigation intervals are separate; a latched fault needs its documented reset after correction.
Does a compressor proving fault mean the compressor is bad?
No. Precedent Symbio 700 proof monitors contactor feedback. Establish the protection, connection, switching, power or feedback fault and actual motor operation before specifying replacement.
Is Carrier T133 proof of low refrigerant charge?
No. The selected HC manual also requires investigation of airflow, frost, refrigerant restrictions and sensing. Use the airside evidence table before assigning charge as the cause.
Can an economizer cool with the compressor off?
Yes. The selected Carrier sequences use available economizer cooling before adding mechanical capacity. Check air temperatures and stage request; use the linked economizer fault-detection guide for damper or sensor disagreements.
Why does the Trane low-pressure fault keep returning after a reset?
The initiating condition remains or returns under load. Precedent G also distinguishes counter recovery by refrigerant. The cause matrix separates automatic recovery from recurring lockout.
Why is the fan running on an R-454B rooftop during refrigerant mitigation?
Equipped Precedent Symbio 700 units intentionally run the supply fan while disabling compressors during mitigation. Read the initiating concentration, sensor or communication diagnostic; preserve protection until recovery prerequisites are satisfied.
Will cycling power clear a Carrier or Trane compressor lockout?
Some current diagnostics clear at power-up, while stored history can remain. Save both, resolve the initiating condition and use the documented reset for the installed controller. A returning diagnostic requires further diagnosis.
What proves the no-cooling repair is complete?
Record demand, availability, command, proof, actual compressor operation, delivered cooling and subsequent faults. The before/after form also records unavailable stages and follow-up conditions.