Reference · 44 codes · v0.1

Error Codes Reference

Diagnostic reference for common residential HVAC fault codes across the major manufacturers. Search by code, description, or cause. Filter by brand, equipment type, or severity.

Always verify against the appliance's installation manual

Codes vary by model year, control board revision, and regional variant. This reference captures the most commonly encountered interpretations, but the installation manual for the specific unit on your truck wins any disagreement.

Manufacturer

Equipment

Severity

44 of 44 codes

Slow flash (1 Hz)·Normal — no fault
Info
GenericMost integrated furnace boardsFurnace

A steady, slow blink of the diagnostic LED indicates the control board is healthy and in standby. No fault history is being reported.

Probable causes

  • ·Normal idle state

First checks

  1. 01No action needed — this is the 'all good' signal.
Rapid flash·Internal control fault
Lockout
GenericMost integrated furnace boardsFurnace

Continuous rapid flashing typically indicates an internal board fault — watchdog reset, flash memory error, or a failed onboard sensor the board can't identify further.

Probable causes

  • ·Control board internal failure
  • ·Water or corrosion contamination on the board
  • ·Transformer secondary voltage out of spec (low-voltage brownout)

First checks

  1. 01Verify 24 VAC at the board's R-C terminals under load (22–28 VAC normal).
  2. 02Inspect the board visually for burnt components, swollen capacitors, or water intrusion.
  3. 03Cycle power. If the rapid flash persists after a cold reset, replace the board.
Steady on (no flash)·Board has no power or has failed completely
Lockout
GenericMost integrated furnace boardsFurnace

LED illuminated but not flashing at all usually means the board has locked up or lost its clock. Sometimes indicates a completely failed board.

Probable causes

  • ·Failed microcontroller or clock crystal on the board
  • ·Firmware hang (rare)
  • ·Stuck relay holding the board in a fault state

First checks

  1. 01Power-cycle the system for 30 seconds.
  2. 02Measure 24 VAC at R-C. If present and LED still solid, replace the board.
13·Limit circuit lockout
Lockout
Carrier / Bryant58MVP, 58MVB, 59SC, similar two-stage + variable speedFurnace

The limit switch has opened more than 3 times during heat calls within a tracking window, or has been open for longer than 3 minutes continuously. The control locks out the call for heat until reset.

Probable causes

  • ·Dirty or clogged return-air filter (most common)
  • ·Closed or blocked supply registers
  • ·Collapsed or undersized ductwork
  • ·Weak or failing blower motor / capacitor
  • ·Actual high-limit switch drift (rare)

First checks

  1. 01Inspect and replace return-air filter.
  2. 02Verify supply registers are open throughout the house.
  3. 03Measure plenum temperature during run with an IR gun — should stabilize below the limit trip point.
  4. 04Check blower motor amp draw against nameplate.
14·Ignition lockout
Ignition
Carrier / Bryant58MVP, 58MVB, 59SCFurnace

Control has detected 4 failed ignition trials within one heat call. Gas valve is locked closed; reset required via thermostat or breaker cycle.

Probable causes

  • ·Dirty flame sensor rod (most common)
  • ·Low or no gas supply
  • ·Failed or weak HSI (hot surface igniter)
  • ·Cracked flame sensor porcelain
  • ·Reversed line polarity
  • ·Ground path issue at burner assembly

First checks

  1. 01Measure flame signal in µA DC — clean rod if below 1.5 µA.
  2. 02Verify 24 VAC at gas valve terminals during ignition trial.
  3. 03Confirm outlet polarity with a plug-in tester.
  4. 04Check inlet gas pressure firing.
21·Gas heating lockout
Lockout
Carrier / BryantInfinity / Performance seriesFurnace

Gas heating component has been locked out after repeated failures. The control considers the heating system untrustworthy until reset and diagnosed.

Probable causes

  • ·Repeat flame-prove failures
  • ·Repeat limit trips
  • ·Gas valve fault

First checks

  1. 01Review any other codes in history — the root cause is usually logged alongside this one.
  2. 02Check 24 VAC to gas valve, flame signal, and limit string.
23·Pressure switch did not open
Sensor
Carrier / BryantInfinity / Performance seriesFurnace

The pressure switch was closed when the control expected it to be open (at start of call for heat, before inducer runs). Indicates a stuck switch or a blocked sensing hose.

Probable causes

  • ·Pressure switch stuck closed (contact welded or mechanically failed)
  • ·Sensing tube pinched, blocked, or waterlogged
  • ·Strong natural draft up the vent holding vacuum on the switch

First checks

  1. 01With inducer off, measure continuity across the pressure switch — should read open. If closed, replace.
  2. 02Inspect the sensing hose for water, soot, or pinches.
25·Reversed line polarity
Wiring
Carrier / BryantInfinity / Performance seriesFurnace

Control has detected that hot and neutral are reversed at the 120V supply. The flame-rectification circuit requires correct polarity to reference neutral and function.

Probable causes

  • ·Miswired outlet or disconnect
  • ·Service entrance polarity issue (less common)
  • ·Temporary power from generator without neutral-ground bond

First checks

  1. 01Test the outlet with a plug-in polarity tester.
  2. 02Correct wiring at whichever junction has the reversal — do not bypass.
31·High pressure switch stuck
Sensor
Carrier / BryantCondensing 90%+ furnacesFurnace

On two-stage condensing models with a high-fire pressure switch, this indicates the high-fire switch is stuck closed or did not open between stages.

Probable causes

  • ·Stuck pressure switch
  • ·Condensate blocking the pressure switch sensing tube
  • ·Restricted flue causing sustained high draft

First checks

  1. 01Check pressure switch with a manometer — measure draft in each stage.
  2. 02Inspect condensate drain trap — a blocked trap can push water back into the pressure-switch hose.
33·Limit switch or flame rollout open
Limit trip
Carrier / BryantInfinity / Performance seriesFurnace

Safety limit switch — primary limit, auxiliary limit, or flame-rollout switch — has opened. A single trip flags this code; repeated trips escalate to a code 13 lockout.

Probable causes

  • ·Airflow restriction (filter, registers, ducts)
  • ·Flame rollout from blocked flue, cracked heat exchanger, or carbon-fouled burner
  • ·Drifted limit switch

First checks

  1. 01If flame rollout is tripped, DO NOT reset until you've investigated the cause.
  2. 02Check airflow: filter, registers, blower operation.
  3. 03Inspect flue for blockage — nests, debris, disconnected sections.
34·Ignition proving failure
Ignition
Carrier / BryantInfinity / Performance seriesFurnace

Ignition trial began but flame was not proven within the timing window. A single trial fail; multiple flag a code 14 lockout.

Probable causes

  • ·Dirty flame sensor rod
  • ·Weak flame signal due to marginal combustion
  • ·Weak HSI
  • ·Low gas pressure

First checks

  1. 01Clean flame sensor rod, re-measure signal.
  2. 02Check HSI resistance and visual condition.
  3. 03Manometer inlet pressure during trial.
1 flash·Ignition lockout
Ignition
Trane / American StandardXV95, XC95, XC80, XR80 seriesFurnace

Control has attempted 3 ignition trials without proving flame and has locked the gas valve closed.

Probable causes

  • ·Flame sensor dirty or coated
  • ·No gas or low gas pressure
  • ·HSI weak or failed
  • ·Reversed polarity

First checks

  1. 01Clean flame sensor with emery cloth.
  2. 02Measure flame signal µA DC.
  3. 03Check inlet gas pressure with manometer.
2 flashes·External lockout — pressure switch fault
Sensor
Trane / American StandardXV95, XC95, XC80, XR80Furnace

Pressure switch failed to close within timing window after inducer started, or was stuck in an unexpected state.

Probable causes

  • ·Blocked or disconnected sensing hose
  • ·Restricted flue or improper venting
  • ·Inducer not pulling sufficient draft (bearings, wheel, capacitor)
  • ·Pressure switch diaphragm failed

First checks

  1. 01Measure actual draft with manometer during inducer run.
  2. 02Compare to pressure switch trip spec on side of switch.
  3. 03Inspect sensing hose for water, soot, kinks.
3 flashes·Primary limit switch open
Limit trip
Trane / American StandardXV95, XC95, XC80, XR80Furnace

High-limit switch has opened, usually due to over-temperature in the heat exchanger plenum.

Probable causes

  • ·Dirty filter (most common)
  • ·Closed registers / ductwork restriction
  • ·Weak blower motor
  • ·Wrong blower speed tap for heat mode

First checks

  1. 01Replace filter.
  2. 02Check blower amp draw against nameplate.
  3. 03Verify blower speed setting is appropriate for system size.
4 flashes·Flame rollout switch open
Limit trip
Trane / American StandardXV95, XC95, XC80, XR80Furnace

Flame rollout switch (manual reset) has opened. Indicates flame escaped the burner tubes, usually from downdraft or blocked flue.

Probable causes

  • ·Blocked or obstructed flue/vent
  • ·Cracked heat exchanger
  • ·Severely carbon-fouled burner
  • ·Negative house pressure pulling flame backward

First checks

  1. 01DO NOT reset without investigation.
  2. 02Inspect vent for blockage or separation.
  3. 03Inspect heat exchanger visually and with combustion analysis.
  4. 04Check house negative pressure with door blower on.
5 flashes·Flame detected with gas valve de-energized
Lockout
Trane / American StandardXV95, XC95, XC80, XR80Furnace

Flame sensor reports flame present when gas valve is commanded off. Control assumes gas valve is leaking internally and locks out.

Probable causes

  • ·Leaking gas valve (most serious — red-flag)
  • ·Flame sensor miswired (sense and ignition leads swapped)
  • ·Shorted flame sensor wire

First checks

  1. 01Perform gas valve drop test with manometer.
  2. 02If valve is leaking, remove from service immediately.
  3. 03Verify flame sensor wiring matches schematic.
7 flashes·Low flame signal
Ignition
Trane / American StandardXV95, XC95, XC80, XR80Furnace

Flame signal µA DC is below minimum threshold but system still operating. This is a warning before outright lockout.

Probable causes

  • ·Oxidized flame sensor rod
  • ·Cracked porcelain insulator
  • ·Poor burner ground path
  • ·Marginal combustion (pressure, orifice)

First checks

  1. 01Clean flame sensor rod.
  2. 02Check ground connection at burner.
  3. 03Measure µA DC with meter in series.
E200·Ignition failure / lockout
Ignition
LennoxG71MPP, SL280, SLP98, EL296VFurnace

Ignition control attempted 3–5 trials without proving flame. Valve locked closed until reset. On Lennox variable-capacity units the code appears on the integrated control display.

Probable causes

  • ·Dirty flame sensor
  • ·Insufficient gas pressure
  • ·HSI fault
  • ·Ground reference lost

First checks

  1. 01Clean flame sensor; measure µA DC.
  2. 02Manometer inlet pressure.
  3. 03Check continuity of HSI at ambient (typical 40–90 Ω).
E201·Low flame current
Ignition
LennoxSL280, SLP98, EL296VFurnace

System is running but flame signal has dropped below the warning threshold. Will escalate to E200 if it drops further.

Probable causes

  • ·Flame sensor oxidation
  • ·Burner soot or carbon
  • ·Reduced combustion quality

First checks

  1. 01Clean flame sensor.
  2. 02Inspect burner for fouling.
  3. 03Consider combustion analysis.
E226·Primary high-limit open
Limit trip
LennoxSL280, SLP98, EL296VFurnace

High-limit switch has opened during operation. Usually an airflow problem.

Probable causes

  • ·Dirty filter
  • ·Closed registers or blocked duct
  • ·Low blower speed
  • ·Seized or failing blower motor

First checks

  1. 01Inspect and replace filter.
  2. 02Verify blower operation and amp draw.
2 flashes·System lockout — failed to detect flame
Ignition
HoneywellS8610U / S8660 IP modules (universal replacements)Universal

Module attempted the configured number of trials (1, 3, or retry depending on strap setting) and never detected flame. Module must be power-cycled to reset.

Probable causes

  • ·Weak or dirty flame sensor
  • ·No gas or low gas pressure
  • ·Weak or failed spark/HSI
  • ·Wrong flame sensor polarity

First checks

  1. 01Measure flame µA DC — Honeywell S86 series needs >0.5 µA to run, >1.0 µA for reliability.
  2. 02Confirm 24 VAC on MV during trial.
  3. 03Inspect ground path at pilot burner / main burner.
3 flashes·Pressure switch / airflow fault
Sensor
HoneywellS8610U / S8660Universal

Pressure switch did not close within the expected window, or opened during run. The module cannot proceed without proof of airflow.

Probable causes

  • ·Inducer not pulling sufficient vacuum
  • ·Sensing hose blocked or disconnected
  • ·Pressure switch failed mechanically

First checks

  1. 01Manometer on the inducer suction side vs. the switch trip spec.
  2. 02Inspect hose end-to-end.
  3. 03Check inducer operation and amp draw.
4 flashes·Flame detected out of sequence
Lockout
HoneywellS8610U / S8660Universal

Module sees flame signal when no flame should be present (pre-purge, post-purge, or with gas valve de-energized). Lockout to protect against a leaking gas valve.

Probable causes

  • ·Gas valve leaking by internally
  • ·Flame sensor wire shorted to hot
  • ·Moisture across sensor terminals

First checks

  1. 01Drop test the gas valve with manometer.
  2. 02Inspect flame sensor wiring for shorts or contamination.
Slow flash LED on valve·Smart Valve normal operation
Info
HoneywellVR8200H / VR8204 / VR8304 Smart ValveUniversal

Smart Valve is powered, ignition control internal to the valve is operational, and system is in standby or running normally.

Probable causes

  • ·Normal state

First checks

  1. 01No action needed.
Rapid flash LED on valve·Smart Valve internal fault
Lockout
HoneywellVR8200H / VR8204 / VR8304Universal

Integrated ignition control inside the Smart Valve has detected an internal fault or repeated ignition failures.

Probable causes

  • ·Failed internal control
  • ·Repeated flame-prove failures (rod, gas, ground)
  • ·Low/high supply voltage

First checks

  1. 01Verify 24 VAC at valve terminals.
  2. 02Reset by removing power for 60 seconds.
  3. 03If fault returns immediately, valve is condemned.
F03·Ignition failure
Ignition
Weil-McLainUltra, Ultra Oil, EvergreenModcon / Tankless

Control attempted ignition sequence and failed to prove flame after configured trial count.

Probable causes

  • ·Dirty flame sensor rod
  • ·Air in gas line (new install or after gas work)
  • ·Gas supply interrupted or too low
  • ·Failed ignition cable / igniter

First checks

  1. 01Clean flame sensor.
  2. 02Bleed air from gas line.
  3. 03Manometer inlet pressure firing.
  4. 04Check igniter and ignition cable.
F04·Flame detected when shut off
Lockout
Weil-McLainUltra, EvergreenModcon / Tankless

Flame signal present with gas valve commanded closed. Control locks out for safety.

Probable causes

  • ·Leaking gas valve
  • ·Flame sensor wire contamination

First checks

  1. 01Drop test gas valve.
  2. 02Inspect flame sensor and wire routing.
F09·Supply temperature sensor fault
Sensor
Weil-McLainUltra, EvergreenModcon / Tankless

The supply (outlet) water temperature sensor (thermistor) is reading out of range — shorted, open, or drifted beyond spec.

Probable causes

  • ·Thermistor failed
  • ·Loose or corroded sensor connector
  • ·Damaged wiring harness

First checks

  1. 01Disconnect sensor, measure resistance. Compare to NTC temp chart (typically 10K at 77°F).
  2. 02Inspect connector and wiring.
F11·Return temperature sensor fault
Sensor
Weil-McLainUltra, EvergreenModcon / Tankless

Return water temperature sensor out of range.

Probable causes

  • ·Thermistor failed
  • ·Wiring issue

First checks

  1. 01Check resistance against temperature chart.
  2. 02Inspect wiring.
Fault 2 (flame sense)·Flame sense lockout
Ignition
LochinvarKnight, WHN, WHB, KHN seriesModcon / Tankless

SMART SYSTEM control has detected loss of flame signal during run, or failed to prove flame within ignition trial window.

Probable causes

  • ·Flame sensor rod fouled
  • ·Weak ignition
  • ·Flame instability (over/under-fire)
  • ·Poor ground path

First checks

  1. 01Clean flame sensor rod.
  2. 02Measure flame µA DC during steady-state run.
  3. 03Check ignition module and cable.
Low Water·Low water cutoff open
Limit trip
LochinvarKnight, WHN, WHBModcon / Tankless

Low water cutoff circuit has opened. Boiler cannot fire until water level is restored and circuit closes.

Probable causes

  • ·Actual low water condition in boiler
  • ·LWCO probe fouled or scaled
  • ·LWCO control failed
  • ·Wiring fault in LWCO loop

First checks

  1. 01Verify boiler water level via sight glass or pressure gauge.
  2. 02Test LWCO per manufacturer procedure.
  3. 03Inspect probe if probe-type LWCO.
45·Control circuit error
Comm
Carrier / BryantInfinity series AC/HPAir conditioning

Outdoor unit has detected a problem with its internal control circuit or communication back to the indoor unit.

Probable causes

  • ·Data wire (A/B bus) miswired or broken
  • ·Failed control board
  • ·Contactor issues

First checks

  1. 01Verify data cable connections at indoor and outdoor units.
  2. 02Check 24 VAC at outdoor control.
81·High-pressure switch open
Limit trip
Carrier / BryantInfinity series AC/HPAir conditioning

High refrigerant pressure has tripped the high-pressure cutout.

Probable causes

  • ·Dirty condenser coil (most common)
  • ·Failing condenser fan motor
  • ·Overcharge
  • ·Non-condensables in refrigerant circuit
  • ·Blocked airflow across the outdoor coil

First checks

  1. 01Clean outdoor coil.
  2. 02Verify condenser fan operation.
  3. 03Check subcooling and refrigerant charge.
82·Low-pressure switch open
Limit trip
Carrier / BryantInfinity series AC/HPAir conditioning

Low refrigerant pressure has tripped the low-pressure cutout.

Probable causes

  • ·Refrigerant leak
  • ·Dirty indoor coil / restricted indoor airflow
  • ·Failed indoor blower
  • ·TXV/EEV stuck closed
  • ·Extremely low ambient operation beyond design

First checks

  1. 01Check indoor airflow.
  2. 02Leak-check the system.
  3. 03Measure superheat and subcooling.
1 flash (outdoor)·Normal operation
Info
Trane / American StandardXR16, XL18i, XV20i AC/HPAir conditioning

Single slow flash on the outdoor control board LED indicates normal standby or normal operation.

Probable causes

  • ·Normal

First checks

  1. 01No action.
3 flashes (outdoor)·Compressor contactor fault
Sensor
Trane / American StandardXR16, XL18iAir conditioning

Outdoor control detects a contactor issue — not pulling in, welded contacts, or mismatch between commanded and sensed state.

Probable causes

  • ·Failed contactor coil
  • ·Welded contactor contacts
  • ·24V control wiring issue

First checks

  1. 01Measure 24 VAC at contactor coil on call for cool.
  2. 02Check contactor contacts visually and electrically.
7·Low outdoor ambient sensor
Sensor
Carrier / BryantInfinity heat pumpsHeat pump

Outdoor ambient temperature sensor is reading out of range. Affects defrost cycle timing and performance.

Probable causes

  • ·Failed outdoor temperature sensor (thermistor)
  • ·Sensor wire disconnected or damaged

First checks

  1. 01Disconnect sensor and measure resistance against NTC curve.
  2. 02Inspect wiring from control to sensor.
Reversing valve fault·Heat pump won't change modes
Sensor
GenericVarious heat pumpsHeat pump

Reversing valve solenoid is energized correctly but valve is not shifting, or shifts only partially (mid-position stall).

Probable causes

  • ·Reversing valve stuck (worn slider)
  • ·Low refrigerant charge — pressure differential insufficient to shift valve
  • ·Failed solenoid coil
  • ·Miswired O/B terminal

First checks

  1. 01Measure 24 VAC at reversing valve solenoid when mode change is commanded.
  2. 02Check solenoid coil resistance.
  3. 03Check refrigerant charge.
  4. 04On heat pumps: verify O/B convention and thermostat setting match.
Flame signal < 0.5 µA·Flame signal below lockout threshold
Ignition
UniversalAll flame-rectification systemsUniversal

Measured flame signal is below the module's minimum threshold. System will not run.

Probable causes

  • ·Dirty / oxidized flame rod
  • ·Cracked porcelain insulator
  • ·Poor burner ground
  • ·Reversed polarity
  • ·Weak flame (combustion issue)

First checks

  1. 01Clean rod with fine emery cloth (not steel wool).
  2. 02Check ground strap at burner.
  3. 03Test outlet polarity.
  4. 04Manometer inlet and manifold pressure.
Audible gas valve hum·Gas valve is being commanded but won't mechanically open
Ignition
UniversalAll gas appliancesUniversal

24 VAC is present at the coil, the coil is pulling in, but the valve isn't opening or is partially opening. Hum without full open = mechanical failure.

Probable causes

  • ·Valve seized internally
  • ·Debris in valve seat
  • ·Aged valve with weakened spring

First checks

  1. 01Verify coil resistance is in spec (typically 40–100 Ω).
  2. 02Verify full 24 VAC under load.
  3. 03If electrical side is fine, replace the valve.
Nonsensical behavior after thermostat work·R and W shorted somewhere in the wiring
Wiring
UniversalAny 24V control systemUniversal

Heat calls on its own, or blown 3-amp control fuse, after thermostat or wiring work. Typically a short between R and W (or R to ground) somewhere in the low-voltage wiring.

Probable causes

  • ·Staple through thermostat cable
  • ·Crushed wire at a junction
  • ·Reversed wire at the thermostat
  • ·Damaged cable after drywall work

First checks

  1. 01Disconnect thermostat. Measure continuity between R and W at the board — should be OL.
  2. 02If continuity is present, short is in the wiring, not the thermostat.
  3. 03Trace the cable route looking for physical damage.