Furnace Fault Codes

Furnace flashing once: what 1 flash means

The number of flashes on a furnace control board is not a standard. Every manufacturer assigns its own meanings, and 1 flash means something different across the 7 control families below.

On an Armstrong it is reverse line voltage polarity, on a Goodman it is lockout after too many failed ignition attempts, and on a Heil it is not used — if you are counting one flash, count again.

These three meanings for one flash have nothing to do with each other — they're unrelated faults, not shades of the same problem, and the count alone won't tell you which applies. Identify the manufacturer before acting on any of them.

Some of the faults listed here are gas-safety faults. Those ones 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, and each carries this warning on its own page. Do not keep resetting a furnace that reports one of them in order to hold 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. The conditions that mean stop now.

1 flash, by manufacturer

Furnace brandWhat 1 means there
Armstrong
1 flash		Reverse line voltage polarity
Goodman
1 flash — Goodman GMS, GMP, GMH, GMV8, GMV9 and Amana AMH, AMV9, AMSS9 series		Lockout after too many failed ignition attempts
Goodman
1 red flash — GMEC96 / AMEC96		Flame sensed when no flame should be present
Goodman
1 amber flash — GMEC96 / AMEC96		Normal operation — first stage heat
Goodman
1 green flash — GMEC96 / AMEC96		Normal operation — low stage cooling
Heil
1 flash		Not used — if you are counting one flash, count again
Rheem
*RC 92+ / *RL 90+		Limit circuit is open
Rheem
Rheem standard gas furnaces		Soft lockout, one hour
York
1 red flash — UTEC / ST		Flame sensed with the gas valve off
York
1 amber flash — ST		Normal operation with a call for heat, Emerson boards

Before counting again

A steady, unbroken blink with no gap at all is not a fault code — it's the board's heartbeat, confirming only that it has power and nothing wrong to report, and it looks nothing like a clean single flash. See the full list of normal, non-fault patterns by brand.

Finding out which brand you have

Check the rating plate inside the furnace cabinet, not the thermostat, to confirm the brand — plenty of furnaces are badged under a name different from whoever actually built the control board. For help finding that plate, or for what to do if your unit shows a number instead of flashing, see reading a furnace fault code.

Other flash counts

What the fix costs

A confirmed bad part usually means a repair bill — see what a control board replacement typically runs, or check the general furnace repair cost guide for what a service call costs overall.

What 1 flash usually points to

Across these boards, 1 flash falls into three different categories: a reversed-polarity wiring problem (Armstrong), an ignition-retry lockout (Goodman, Rheem's standard board), or a flame-sensing fault where the sensor reports flame with the gas valve closed (York's red flash). On boards that use color, check whether the flash is red — a fault — or amber or green, which on the GMEC96/AMEC96 and York Emerson boards just marks a normal heat or cooling stage. That split between wiring, ignition, and flame-sensing is unique to 1 flash; 2 flashes below is almost entirely about the pressure switch instead.

Sources

Each single-flash meaning above traces back to the manufacturer service document linked on its code page:

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.