No power to furnace (control board not lit)
No power to furnace (control board not lit).
Normal operation (standby, no heat demand)
Normal operation (standby, no heat demand).
Normal operation (heat cycle active)
Normal operation (heat cycle active).
Control board fault (malfunction)
Control board fault (malfunction).
Ignition failure (burner failed after retries)
Ignition failure (burner failed after retries). The furnace may lock out for safety, blowing only cold air.
Pressure switch failure (venting/inducer problem)
Pressure switch failure (venting/inducer problem). The pressure switch did not close or reopen appropriately.
High-limit switch open (furnace overheated)
High-limit switch open (furnace overheated). This safety limit trips when temperature is too high, usually from airflow issues.
Flame sensed with gas valve off (unexpected flame)
Flame sensed with gas valve off (unexpected flame). This could indicate a gas valve leaking or stuck open, or a control board issue.
Line polarity or grounding issue
Line polarity or grounding issue. The control board senses incorrect electrical polarity or a grounding problem.
Gas valve circuit error
Gas valve circuit error. The gas valve is not opening or closing correctly, or its circuit has failed.
Weak flame signal (poor flame sense)
Weak flame signal (poor flame sense). The burner’s flame is present but not detected strongly by the sensor (could extinguish).
Igniter problem
Igniter problem. The hot surface ignitor failed to light the burners or is not drawing the correct current.
Flame problem sensed after ignition (flame instability)
Flame problem sensed after ignition (flame instability). This indicates the burner flame itself is abnormal or not sustaining properly.