Furnace Fault Codes

Furnace flashing 4 times: what 4 flashes means

The number of flashes on a furnace control board is not a standard. Every manufacturer assigns its own meanings, and 4 flashes means something different across the 8 control families below.

On an Armstrong it is low flame signal — check the flame sensor, on a Goodman it is high limit switch is open — the furnace is overheating, and on a Heil it is limit switch string is open.

Three different brands, three unconnected problems — a count of four doesn't mean one fault with small variations. Know which brand you're working on before you touch anything.

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.

4 flashes, by manufacturer

Furnace brandWhat 4 means there
Armstrong
4 flashes		Low flame signal — check the flame sensor
Goodman
4 flashes — Goodman GMS, GMP, GMH, GMV8, GMV9 and Amana AMH, AMV9 series		High limit switch is open — the furnace is overheating
Goodman
4 red flashes — GMEC96 / AMEC96		Open limit switch
Goodman
4 amber flashes — GMEC96 / AMEC96		Cooling called with no fan call
Heil
4 flashes		Limit switch string is open
Rheem
*RC 92+ / *RL 90+		Lockout after five failed ignition attempts
Rheem
Rheem standard gas furnaces		Pressure switch failure — switch remains closed
Trane
4 flashes		Open high limit device
York
4 red flashes — UTEC / ST		Open high limit switch
York
4 amber flashes — UTEC / ST		Cooling called with no fan call

Before counting again

A blink that never pauses, with no grouping of four or anything else, just means the board is alive and fault-free — it's a heartbeat, not a diagnostic code. Check the by-brand rundown of normal, non-fault patterns to compare.

Finding out which brand you have

The thermostat won't say who made the furnace; the rating plate inside the cabinet will, and it's frequently a different name from the company behind the control board. Reading a furnace fault code explains where to find that plate and what a numeric display, instead of flashes, means.

Other flash counts

What the fix costs

Diagnosis pointing to a bad component means pricing a fix next — control board replacement cost if that's the part, or the general furnace repair cost guide for typical service-call rates.

What 4 flashes usually points to

4 flashes moves to the high-limit switch on most of these boards — Goodman, Heil, Trane and York all report it here, which points to the furnace overheating: a blocked filter, closed supply registers, or a blower not moving enough air. Rheem's standard board instead reuses 4 flashes for the pressure-switch failure opposite the one it reports at 2 flashes, and its *RC/*RL board uses 4 flashes for an ignition lockout. Armstrong ties 4 flashes to a weak flame-sensor signal rather than the limit circuit. Check airflow first if your board falls in the limit-switch group — a different fix path from the pressure-switch and ignition faults that dominate the counts on either side.

Sources

All four-flash meanings above come straight from the manufacturer service sheets linked on their code pages:

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.