How do you fix Goodman flashes?

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. Full steps and safety notes are on this page.

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.

Do the simple checks below, then stop. The simple checks listed above are genuinely homeowner work and worth doing first. Beyond them, this fault involves 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. If the simple checks do not resolve it, stop there and call a licensed technician rather than resetting the furnace repeatedly to keep the heat on. 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.

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.

All codes for this manufacturer

Goodman and Amana furnace fault codes — every code, with what differs between models.

Not sure this is your brand?

The same count means something else on other furnaces. What 4 flashes means on every brand puts them side by side.

Models this code appears on

From the manufacturer's per-model database. If your model is not listed, this code may not exist on your board, or it may mean something different — check the model page.

The same fault on other control families

The open high limit is universal furniture: Carrier 33 covers it (folding rollout into the same circuit, a Carrier quirk), Allied shows seven flashes, Lennox alert 250 names the primary limit, and ICP uses four flashes as Goodman does. The limit switch code hub tracks seventeen versions across eleven families and hammers the point that matters: the switch is almost never the fault — it is the witness to an airflow problem.

What the fix costs

The distribution here is friendly: most four-flash stories end with a filter, which is to say a few dollars and no visit. When the trip repeats on a clean filter, the repair cost guide prices the escalation — limit switch work at $150–350 with the explicit caveat that the job is finding why it tripped, and blower diagnosis beyond that. On models of this era fitted with PSC blowers, the guide's capacitor rule applies in full: a blower that hums without spinning is a $150–350 capacitor fix far more often than a $400–700 motor, and the test takes minutes. The number to refuse is a bare switch swap with no airflow investigation attached — that is paying $250 to reset a smoke alarm.

DIY or call a pro?

This code owns the most legitimate DIY in furnace-dom: replace the filter, open every register, clear the returns, and let the furnace cool before it retries. If the trip never returns, you fixed it for pocket change. The professional line is repetition — a second or third trip on a clean filter means the restriction or the blower needs measuring, and limit trips are not a condition to live with: each one bakes the heat exchanger, and cracked heat exchangers are both the $1,000–2,500+ line item and the carbon monoxide conversation. Never bypass or 'stretch' a limit switch, and treat a furnace that needs its limit reset weekly as a furnace that is telling you something expensive quietly.

What this looks like

From the outside, this usually shows up as the furnace heats then trips out on high limit — that walkthrough covers what to check in order, and when it stops being a homeowner job.

This can also show up as a burning or gas smell from the unit — that page sorts the normal smell from the ones that mean shut it down.

Sources

Goodman Manufacturing — Diagnostic Code Identification System (model-specific fault code lookup)

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.