Goodman furnace 4 red flashes: Open limit switch
Four red flashes means a limit switch has opened because the furnace overheated. The cause is almost always restricted airflow, and the most common single reason is a dirty filter.
Applies to: Goodman GMEC96 and Amana AMEC96 two-stage gas furnaces with a three-colour diagnostic LED.
What it means
The limit switch opens when air passing over the heat exchanger is not carrying heat away fast enough. The control shuts off the gas and runs the blower to cool things down. This is protective rather than incidental: a heat exchanger that repeatedly overheats will eventually crack, and a cracked heat exchanger can leak combustion products, including carbon monoxide, into the air circulating through the house. The switch is usually working correctly and reporting something true, which is why replacing it is almost never the answer.
Most common causes
- A clogged air filter — by a wide margin the most common cause.
- Closed, blocked, or covered supply registers, or too many closed at once.
- A blocked return air grille.
- Undersized or restrictive ductwork, or a duct that has come apart or collapsed.
- A failing blower motor, or a blower wheel packed with dust.
- Blower speed set too low for the furnace's output.
- Overfiring — manifold pressure set too high for the airflow available.
How to diagnose it
- Confirm the colour is red. Four amber flashes on this control signals a cooling call without a fan call, which is a configuration issue rather than an overheat.
- Check the air filter first. Grey, matted, or opaque to light means you have very likely found it.
- Walk the house and count closed or blocked supply registers, and check the return grille.
- Note whether the blower runs at all — if the furnace fires and the blower never starts, overheating is certain.
- Note whether it only happens on long runs, which is the marginal-airflow signature.
How to fix it
- Replace the air filter with the correct size and type and check it monthly during the heating season.
- Open all supply registers and clear the return grille.
- Blower motor work, blower wheel cleaning, duct repair, speed changes, and manifold pressure adjustment are technician tasks.
- If the code returns after a filter change, stop resetting it and have the furnace inspected. Repeated overheating is what damages heat exchangers.
Notes
Worth taking seriously despite its usually trivial cause. The switch exists because sustained overheating cracks heat exchangers, which is the most dangerous fault a gas furnace can develop.