Condensing gas furnace with PVC vent pipes and metal ductwork in a utility room

Generic photo: Wtshymanski · CC BY-SA 4.0

Furnace control boards: last-fault memory, fault recall and power-cycle rules (Carrier/Bryant, ICP, Goodman/Amana, Rheem/Ruud, Trane, Lennox)

Furnace boards show the current state on an LED or a 7-segment display, and many also keep a fault history. Some keep it only until power is removed; one Carrier-family description says stored codes clear on power loss or about 48 hours after the fault. Others have a fault-recall push button or recall terminal that replays recent faults. Read the LED through the door sight glass first; removing the blower door usually opens the door switch and drops power to the board.

Cross-checked ~15 min easyBrands: Carrier, Bryant, Payne, Heil, Tempstar, Comfortmaker, Goodman, Amana, Rheem, Ruud, Trane, American Standard, Lennox

Before you start

Gas valve, gas pressure, burners and venting/combustion faults are blocked: if the code points there or you smell gas, shut the gas off and refer to a licensed gas/HVAC technician
The board has 120 V on it; live readings are caution, insulated probes only
Never jumper safety switches (limit, rollout, pressure switch) to make a code go away

Diagnostic steps

Unlock all 7 steps with expected values

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