Armstrong furnace 12 flashes: Flame sensed without the gas valve energised

Twelve flashes means the control is detecting a flame while the gas valve is not energised. Either gas is burning when it should not be, or the sensing circuit is reporting a flame that is not there.

Applies to: Allied Air Enterprises A80UH1E and 80G1UHE gas furnaces with the -02 integrated control, sold as Armstrong Air, AirEase, Concord and Ducane.

What it means

The control watches the flame sensor continuously, not only during the burn. Detecting flame with the gas valve de-energised produces this code. It is treated seriously because one of the two possible causes is a gas valve that is not fully closing — gas entering the combustion chamber outside a controlled cycle. The other is electrical: a short in the flame sensor wiring produces a signal the control cannot distinguish from a real flame. Since it cannot tell them apart, it assumes the dangerous one, which is correct behaviour for a control managing a gas appliance.

Most common causes

A gas valve leaking through or not closing fully.
A slow-closing gas valve, where flame lingers after the cycle ends.
A short to ground in the flame sensor wiring or at the sensor.
A chafed or damaged sensor wire touching the cabinet.
Residual flame on a burner after the main flame is cut.
A failed control misreading the flame signal.

How to diagnose it

Note any smell of gas and act on it immediately rather than continuing to diagnose.
Do not open the burner compartment while the furnace is powered. There is no safe homeowner check for this fault.
Note whether the inducer runs continuously without the furnace heating, which can accompany this condition.
Count carefully. Twelve is at the long end of this code set and easy to miscount.

How to fix it

If you smell gas: leave the building, operate no electrical switches, and call your gas utility's emergency line or the fire service from outside. Do not go back in to investigate.
With no gas smell, turn the furnace off at its service switch and leave it off until a licensed technician has seen it.
This fault is entirely technician work. It involves the gas valve and the flame-sensing circuit.
Do not reset repeatedly to see whether it clears. If a valve is passing gas, each reset is another opportunity for it to accumulate.

Notes

The wiring-short version is far more common than a leaking gas valve, and a technician usually finds it quickly. That does not make it safe to assume — the control cannot distinguish them, and neither can anyone without test equipment at the valve.

All codes for this manufacturer

Armstrong Air, AirEase, Concord and Ducane furnace codes — every code, with what differs between models.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

Allied Air Enterprises — A80UH1E and 80G1UHE Installation Instructions 507330-03, Table 7: Diagnostic Codes for -02 Control

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.