HVAC & VRF Fault Codes
Split DX, heat pump, rooftop, and VRF / mini-split fault codes for Carrier, Trane, Lennox, Goodman, Rheem, York, Mitsubishi, Daikin, LG, Fujitsu, and more.
The system has lost communication between the control board and the indoor blower motor.
- Failed motor serial interface
- Loose communication wire
- Faulty indoor control board
- Check wiring harness between motor and control board
- Verify 24VAC and high voltage power to the motor
- Inspect connectors for corrosion or loose pins
The system has detected a general failure or operation error within the indoor blower motor.
- Seized motor bearings
- Internal motor controller failure
- Blower wheel imbalance
- Check for obstructions in the blower wheel
- Verify motor power supply
- Ensure motor shaft rotates freely
The furnace control has locked out blower operation due to a persistent motor fault.
- Repeated motor start failures
- Overheated blower motor
- Faulty control board relay
- Power cycle the furnace to attempt a reset
- Check motor wiring and ground
- Check for any physical blockages in the fan housing
The heater output was detected as active even though the control board had not energized it.
- Welded sequencer or contactor
- Short circuit in heater control wiring
- Failed indoor control board
- Check for shorted heating element contactors
- Inspect control board for stuck relays
- Verify voltage at heater terminals while idle
The furnace limit switch has opened and remained open, leading to an operation lockout.
- Restricted airflow
- Defective limit switch
- Over-fired furnace
- Check for dirty air filters
- Inspect all supply registers and return grilles for blockages
- Verify blower motor speed and operation
The furnace failed to ignite the gas burner or prove the flame after several attempts.
- Dirty flame sensor
- Faulty igniter
- Low gas pressure
- Defective gas valve
- Verify gas supply valve is open
- Check condition of the igniter
- Inspect the flame sensor for soot or oxidation
The system has locked out gas heating operation due to repeated malfunctions.
- Repeated ignition failures
- Continuous limit circuit trips
- Gas valve circuit malfunction
- Check gas supply and manifold pressure
- Verify proper venting and combustion air
- Review previous fault codes in history
A flame signal was detected by the control board while the gas valve was supposed to be closed.
- Leaking gas valve
- Faulty flame sensor
- Control board sensing error
- Check for a leaking gas valve
- Inspect flame sensor wiring for shorts to ground
- Ensure the burner is not actually lit
The inducer pressure switch stayed in the closed position when it was expected to be open at start-up.
- Stuck pressure switch
- Miswired control circuit
- Obstruction in the pressure switch tubing
- Check for a stuck or shorted pressure switch
- Verify wiring to the pressure switch
- Check for debris holding the switch closed
The 24-volt fuse on the furnace control board has blown, indicating a low-voltage short.
- Shorted contactor coil
- Pinched thermostat wire
- Faulty accessory component
- Inspect low voltage wiring for shorts to ground
- Check contactor coils for shorts
- Verify thermostat wiring integrity
The limit circuit has opened, and the system is operating in high heat only as a result.
- Airflow restriction
- Faulty limit switch
- High static pressure
- Check for dirty air filters
- Verify all dampers are open
- Check blower motor RPM and static pressure
An internal software or hardware failure has occurred within the furnace control board.
- Internal board component failure
- Electrical interference/noise
- Memory failure on board
- Reset power to the furnace control
- Check for proper grounding
- Verify secondary voltage is within range
Unexpected voltage was detected on the compressor circuit while the unit was in standby mode.
- Stuck or welded contactor
- Shorted wiring
- Control board relay failure
- Check contactor for welded contacts
- Inspect outdoor unit wiring
- Verify control signals from the indoor unit
The outdoor unit has locked out high-capacity operation due to a performance or electrical issue.
- Low refrigerant charge
- Faulty compressor start components
- Internal compressor thermal overload
- Check compressor capacitor
- Verify refrigerant levels
- Check compressor winding resistance
The system has detected that the airflow is restricted, which can lead to inefficient operation.
- Dirty air filters
- Closed supply vents
- Blocked return air duct
- Check air filters for dirt
- Inspect all vents for closure
- Check ductwork for obstructions
The system has shut down to protect components due to excessively high refrigerant pressure.
- Dirty outdoor coils
- Faulty outdoor fan motor
- Debris blocking airflow in the outdoor unit
- Check outdoor unit for dirt and debris
- Verify the outdoor fan motor is running
- Check for coil obstructions
The system has shut down because the refrigerant pressure has dropped below the safe operating threshold.
- Low refrigerant charge
- Refrigerant leak in the lines or coils
- Check refrigerant levels
- Inspect system for visible leaks
The system is unable to read the outdoor temperature correctly due to a sensor malfunction.
- Faulty outdoor temperature sensor
- Damaged sensor wiring
- Inspect the outdoor temperature sensor
- Check sensor wiring for damage or loose connections
There is a failure in communication between the indoor and outdoor control boards.
- Loose wiring connections
- Faulty communication module
- Interrupted control signal
- Check all wiring connections between units
- Reset the system by cycling power for 10 minutes
The system has detected a low refrigerant level which is impacting heating or cooling performance.
- Refrigerant leak
- Undercharged system from installation
- Look for visible oil or refrigerant leaks
- Check refrigerant levels with gauges
The system has detected a failure or malfunction with the outdoor fan motor operation.
- Faulty fan motor
- Failed fan capacitor
- Motor overheating
- Inspect the outdoor fan for physical obstructions
- Check if the fan motor is overheating
The system is powered and operational but currently has no active demand for heating or cooling.
- Normal operation
- Thermostat at setpoint
- No call for service
- Check thermostat settings
- Verify communication with the indoor unit
The outdoor unit is currently operating in its first stage of compressor capacity.
- Normal operation for low-demand cooling or heating
- Verify compressor operation
- Observe system performance
The outdoor unit is currently operating in its second (high) stage of compressor capacity.
- Normal operation for high-demand cooling or heating
- Verify compressor operation
- Observe system performance
The communication link between the outdoor control board and the rest of the system is broken.
- Loose wire connections
- Broken or shorted communication wires
- Electrical interference
- Failed control board
- Check ABCD wiring connections
- Inspect for damaged communication wires
- Reset power to the entire system
The control board has detected a missing or incorrect model plug for this unit.
- Missing model plug
- Model plug not fully inserted
- Incorrect model plug installed
- Verify model plug is seated correctly
- Check that the model plug matches the unit model
The high pressure switch has opened due to excessive refrigerant pressure.
- Dirty outdoor coil
- Outdoor fan motor failure
- Refrigerant overcharge
- Non-condensables in system
- Check outdoor coil for debris
- Verify outdoor fan is operating
- Check for closed service valves
The system has tripped due to low refrigerant pressure or high compressor discharge temperature.
- Refrigerant leak
- Dirty indoor filter
- Faulty indoor blower motor
- Restricted expansion valve
- Check refrigerant charge levels
- Inspect indoor air filter and coil
- Check for indoor blower operation
The incoming line voltage is below the minimum threshold for safe unit operation.
- Utility company brownout
- Undersized power supply wiring
- Loose electrical connections
- Measure line voltage at the disconnect
- Check for loose power connections
The control board is not detecting high-voltage power at the outdoor unit.
- Tripped circuit breaker
- Blown fuses
- Broken power wire
- Check main circuit breaker
- Check outdoor disconnect fuses
- Verify wiring from disconnect to unit
The sensor measuring outdoor ambient temperature is malfunctioning.
- Faulty temperature sensor
- Disconnected or cut sensor wire
- Corroded connector
- Check sensor wiring and plug
- Measure sensor resistance (ohms)
The sensor measuring outdoor coil temperature is malfunctioning.
- Faulty coil sensor
- Broken sensor wire
- Moisture in sensor connector
- Check sensor wiring and plug
- Measure sensor resistance (ohms)
The outdoor and coil sensors are reporting values that are inconsistent with expected operating ranges.
- Faulty sensors
- Extreme environmental conditions
- Wiring short across sensor circuits
- Inspect both sensor connections
- Verify sensors are mounted correctly
The compressor thermal protector has opened during low-stage operation.
- Low refrigerant charge
- Compressor mechanical strain
- Internal compressor failure
- Check compressor for overheating
- Verify refrigerant levels
The compressor thermal protector has opened during high-stage operation.
- High compressor head pressure
- Mechanical compressor failure
- Inadequate compressor cooling
- Check compressor for overheating
- Verify refrigerant levels
The control board detects that the compressor contactor remains closed when it should be open.
- Welded contactor points
- Mechanical binding of contactor
- Faulty relay on control board
- Inspect contactor for welded contacts
- Verify contactor coil voltage
The control board has energized the contactor, but no current is detected moving to the compressor.
- Broken wire to compressor
- Open internal compressor winding
- Failed contactor
- Check wiring between contactor and compressor
- Check compressor internal protector (continuity)
The low stage thermal cutout has occurred three times, triggering a hard lockout.
- Repeated compressor overheating
- Chronic low refrigerant charge
- Defective compressor
- Examine compressor history
- Evaluate system charge and conditions
The high stage thermal cutout has occurred three times, triggering a hard lockout.
- Repeated high pressure/heat issues
- Severe airflow restrictions
- Compressor failure
- Examine compressor history
- Check high side system pressures
The low pressure switch has tripped three times, triggering a hard lockout.
- Significant refrigerant leak
- Failed indoor blower motor
- Severe system restriction
- Perform leak search
- Check for air flow restrictions
The high pressure switch has tripped three times, triggering a hard lockout.
- Repeated high pressure events
- Blocked condenser airflow
- System overcharge
- Clean outdoor coil
- Verify outdoor fan motor operation
The number of indoor units detected by the system does not match the quantity specified by the S12 and ENC3 rotary switch settings.
- Incorrect manual setting of the IDU quantity switches
- Communication failure between the outdoor unit and one or more indoor units
- Indoor unit power supply interruption
- Verify the position of the S12 and ENC3 switches on the outdoor unit
- Confirm the actual number of indoor units installed in the system
- Check that all indoor units are powered on and properly addressed
The system has triggered protection for the compressor or detected a temperature fault.
- Compressor fault
- Compressor temperature protection
- Check compressor temperature
- Inspect compressor electrical connections
The Intelligent Power Module (IPM) has failed or is malfunctioning.
- Bad Intelligent Power Module (IPM)
- Inspect IPM for heat damage
- Verify electrical connections to the module
The fuel tank level is low and requires replenishment to ensure continuous operation.
- Empty fuel tank
- Fuel leak
- Faulty fuel sensor
- Check fuel tank level
- Verify fuel gauge operation
The engine oil level has dropped below the safety threshold.
- Excessive oil consumption
- Oil leak
- Sensor failure
- Check engine oil dipstick
- Inspect for oil leaks
Engine coolant level is insufficient for safe operation.
- Cooling system leak
- Evaporation over time
- Failed radiator cap
- Check coolant overflow tank
- Inspect radiator for leaks
The system detects a low level of refrigerant in the circuit.
- Refrigerant leak
- Improper charge during service
- Sensor malfunction
- Check refrigerant sight glass
- Inspect for oily residue indicating leaks
The engine oil pressure is too low, which can cause internal damage; the unit may attempt to restart.
- Low oil volume
- Failed oil pump
- Worn engine bearings
- Check engine oil level
- Inspect oil filter for blockage
Engine coolant temperature has exceeded the safety operating limit.
- Blocked radiator
- Stuck thermostat
- Water pump failure
- Check radiator for debris
- Check fan belt tension