The control senses flame when there is no call for heat. This can indicate an unsafe combustion or gas-control condition.
Professional Required
Flame sensed with no call for heat (Improper flame)
Induced draft and circulator blowers run continuously; no furnace operation; LED displays E4; thermostat shows ‘Check Furnace’
Short to ground in flame‑sense circuit; lingering burner flame; slow closing gas valve
Stop using the furnace and contact a qualified technician promptly. Do not attempt to adjust the gas valve or ignition controls.
Flame sensor, flame‑sensor wiring, gas valve
Do not restart the furnace until a qualified HVAC technician has investigated the safety fault.
If you smell gas, leave the area immediately and contact your gas utility or emergency services from a safe location. Never bypass furnace safety switches. Internal furnace servicing should be performed by qualified personnel.
Learn more about the furnace components and operating conditions commonly associated with diagnostic error codes: