Furnace Error Codes & Troubleshooting
Look up error codes from top furnace brands. Understand what they mean and when to call an emergency technician.
Ignition Lockout
Furnace has attempted ignition 3 times and failed, triggering a safety lockout. No heat until the system is reset and the root cause is addressed.
Pressure Switch Stuck Open
The pressure switch detects inadequate draft inducer vacuum and prevents the furnace from firing to ensure safe exhaust venting.
Limit Switch Open
The high-temperature limit switch has detected excessive heat in the plenum and shut down the furnace to prevent overheating.
Flame Sensor Circuit Failure
The flame sensor is not detecting a flame after the gas valve opens, causing the system to shut down the burner.
Gas Valve Circuit Fault
The control board detects an electrical fault in the gas valve circuit and will not attempt ignition.
Low Fire Pressure Switch (Carrier/Bryant)
Low fire pressure switch not closing during modulated heat operation, specific to Carrier and Bryant two-stage and modulating furnaces.
Limit Circuit Lockout (Lennox)
Lennox furnace has detected the limit switch opened 5 times in a single call for heat, locking out the system.
Ignition Failure (Lennox)
Lennox-specific error indicating the furnace failed to ignite after multiple attempts and locked out.
Limit Switch Fault (Lennox)
Lennox furnace limit switch is open at startup or opens during operation, preventing normal function.
Gas Valve Wiring (Lennox)
Lennox control board detected a wiring error in the gas valve circuit and will not proceed with ignition.
Blower Motor Overload (Lennox)
Lennox furnace blower motor is drawing excessive current or has an open circuit, triggering overload protection.
Pressure Switch Stuck Closed (Generic)
Pressure switch is stuck in the closed position, indicating the switch didn't open when the draft inducer shut off.
Limit Switch Open (Generic)
Primary limit switch has opened during operation, indicating the furnace is overheating or the switch has failed.
Flame Sensed Without Call for Heat
The flame sensor detects a flame when the gas valve should be closed, indicating a stuck valve or sensor issue.
Gas Valve Relay Short
The control board detects a short circuit on the gas valve relay circuit and will not energize the valve.
Blower Motor Fault
The furnace control board has detected a fault with the blower motor circuit and cannot operate the fan.
Line Voltage Error
The control board detected incorrect line voltage, either too high or too low, affecting all furnace operations.
Flame Amplifier Fault
The flame sense circuit is not generating enough microamp signal for reliable flame detection.
Control Board Communication Error
The furnace control board has lost communication with integrated accessories or components.
Draft Inducer Motor Fault
The draft inducer motor is not running or not reaching proper speed during the startup sequence.
Low Gas Pressure
The gas supply pressure is too low for proper furnace operation, affecting burner flame characteristics.
Rollout Switch Tripped
The rollout switch has detected flames or excessive heat escaping the burner compartment and shut down the furnace.
Inducer/Blower Communication Error
Communication error between the variable-speed inducer or blower motor and the furnace control board.