Goodman furnace 4 flashes: High limit switch is open — the furnace is overheating
Four flashes means a temperature limit switch has opened because the heat exchanger got too hot. Almost always the cause is not enough air moving through the furnace, and the most common reason for that is a dirty filter.
Applies to: Goodman GMS, GMP, GMH, GMV8, GMV9 and Amana AMH, AMV9 series gas furnaces with a single red diagnostic LED. On Goodman GMSS9 and Amana AMSS9 the same flash count covers the primary or auxiliary limit circuit being open.
What it means
The limit switch is a temperature-sensitive safety device that opens when the air passing over the heat exchanger is not carrying heat away fast enough. When it opens, the control shuts off the gas and runs the blower to cool the furnace down. This is protective: a heat exchanger that repeatedly overheats will eventually crack, and a cracked heat exchanger can let combustion products, including carbon monoxide, into the air being circulated through the house. So this code should be read as the furnace protecting itself from a real airflow problem, not as a faulty switch. The switch is usually working correctly and reporting something true.
Most common causes
- A clogged air filter, which is by a wide margin the most common cause.
- Closed, blocked, or covered supply registers, or too many registers closed at once.
- A blocked return air grille — furniture, a rug, or a filter that has collapsed into the opening.
- Undersized or restrictive ductwork, or a duct that has come apart or collapsed.
- A failing blower motor, or a blower wheel packed with dust so it cannot move its rated air.
- The blower speed set too low for the furnace's output.
- An overfired furnace — manifold pressure set too high, delivering more heat than the airflow can carry.
- A genuinely failed limit switch, which is possible but much less likely than the airflow causes.
How to diagnose it
- Check the air filter first. If it is grey, matted, or you cannot see light through it, you have very likely found the fault.
- Walk the house and count closed or blocked supply registers. Closing off rooms to save money is a frequent and unintuitive cause.
- Check the return air grille is clear and unobstructed.
- Note whether the blower runs. If the furnace fires and the blower never starts, the overheat is guaranteed and the blower circuit is the fault.
- Note whether it happens only on long runs. A furnace that heats normally at first and trips after twenty minutes is a classic marginal-airflow symptom.
How to fix it
- Replace the air filter with the correct size and type, and check it monthly during the heating season. Fitting a filter far more restrictive than the system was designed for causes this fault as reliably as a dirty one.
- Open all supply registers and clear the return grille.
- A blower motor, blower wheel cleaning, duct repair, blower speed change, or manifold pressure adjustment is technician work.
- If the fault returns after the filter is changed, stop resetting it and have the furnace looked at. Repeated overheating is what damages a heat exchanger, and heat exchanger damage is a carbon monoxide risk rather than a comfort problem.
Notes
This code is worth taking seriously even though its usual cause is trivial. The switch exists because sustained overheating cracks heat exchangers, and a cracked heat exchanger is the single most dangerous fault a gas furnace can develop. A furnace that trips its limit repeatedly should be inspected rather than reset.