Flame detected when no flame should be present

What is going on

Control sees flame with no call for heat, so the inducer and circulator blower keep running.

Likely causes

• Short to ground in the flame sense circuit, which looks like flame to the control
• Lingering burner flame after the gas should be off
• Slow closing gas valve

Check in this order

1. Power off before any repair.
2. Look at the burners with the thermostat satisfied. Flame still there means gas is passing or lingering.
3. Correct the short at the flame sensor or in its wiring, because the wiring can ground out on the chassis.
4. Verify proper operation of the gas valve and that it closes fully and promptly.
5. A gas valve that will not shut off goes to a licensed gas tech.
6. Retest through a few full cycles.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)