Goodman, Amana GMSS9 / GCSS9 error code 4 flashes
Primary or auxiliary limit circuit open
Goodman is the platform name, and your unit may not say it
Amana AMSS9 / ACSS9 is the same board with a different badge, so this page covers all of them. The fault, the checks and the parts are identical, which is why there is one page rather than one per brand.
Which models this applies to. Applies to every legacy flash furnace in this group: GMV8 / GMV9, GCH / GDH / GME / GMH, GDS / GHS / GMS / GKS, and GMN / GMP / GMT. A high limit trip on a GMN and the same trip on a GMV9 are the same fault, because both use the same limit switch and the same blower.
Quick answer
On Goodman GMSS9 / GCSS9 (2000s), 4 flashes is a fault. It means primary or auxiliary limit circuit open. A first thing to check: Dirty or undersized furnace filter.
- Applies to
- Goodman GMSS9 / GCSS9
- Goodman and Amana GMSS9, GCSS9, AMSS9, ACSS9 flash codes · Furnace · 2000s
- Type of code
- Fault
- Urgency: Needs attention soon
- First safe check
- Replace the filter. Not the cheap one, and not a slice of cardboard. A 1 inch filter on a system rated for 4 inches media will trip this switch repeatedly.
What 4 flashes means on this model
Airflow through the furnace dropped below what the heat exchanger needs, so the high limit switch opened and the control shut the burners down. It is protecting the exchanger from being burned through, and it will keep doing so until airflow is restored.
Symptoms you will notice
- Furnace runs, then stops after a few minutes
- Warm air briefly, then nothing
- Airflow is weak or the rooms stay cold
- Code returns after the unit cools down and cools off again
Common causes
The code tells you what the control detected, not which part failed. These are the causes we see most often, roughly in order of likelihood:
- Dirty or undersized furnace filter
- Closed or missing registers, or registers blocked by furniture
- Return air grille blocked or undersized for the system
- Blower wheel packed with dust and no longer moving the designed air volume
- Furnace installed in a closet or tight space with poor combustion air
- Duct restriction, disconnected duct, or a collapsed flex run
- Failing blower motor running slow under load
What you can safely check
These are homeowner-level checks. None of them need you to open a live electrical compartment or work on gas.
- Replace the filter. Not the cheap one, and not a slice of cardboard. A 1 inch filter on a system rated for 4 inches media will trip this switch repeatedly.
- Walk every supply register and confirm air is actually moving out of each one.
- Look at the return grille in the main hallway or living room. It is often behind a sofa.
- Turn the system on and hold your hand at the supply registers. Weak or uneven air points at the blower or the ducts.
Do not do these
- ×Do not bypass the high limit switch. It is the last thing standing between a failed blower motor and a burned heat exchanger.
- ×Do not leave the filter slot open or install a filter rated for a thinner media than the cabinet calls for.
When to call a technician
Book a technician, do not keep resetting it
This fault will not clear on its own. Every power cycle gives the control one more failed attempt, and repeated attempts are what turn a cheap part replacement into a seized or contaminated system.
- This is a fuel-burning appliance, so treat anything involving a burner, gas valve or flue as technician work. If you smell gas at any point, leave the property and call the gas utility from outside rather than continuing to diagnose.
- If the appliance has stopped mid-cycle, is making a noise you have not heard before, or smells burnt, stop reading here and call. Those are symptoms of something the fault code does not describe well enough.
Filters, registers and grille access are all homeowner work. A blower motor is technician work on a sealed combustion unit.
What it costs to fix
Parts
1 inch pleated filter $8 to $20. 4 inch media filter $20 to $40. Blower motor $250 to $500. Capacitor $15 to $40.
Labor
Filter and register work is often free if you do it. Blower motor replacement is two to four hours because of the burners and gas train in the way.
DIY or technician
Filters, registers and grille access are all homeowner work. A blower motor is technician work on a sealed combustion unit.
Typical US part and labor ranges, 2026. They vary by region, model and contractor, so treat them as a planning figure, not a quote.
Scope of this code
- Code table
- Goodman and Amana GMSS9, GCSS9, AMSS9, ACSS9 flash codes
- Models in scope
- Goodman GMSS9 / GCSS9, 2000s
- Checked
- Confirm on your own unit
- Find the model number on the data plate, then check this code in the service manual for that model. Manufacturers revise code tables between model years, and a revised board can renumber a fault.