Goodman GMSS960803BN Error Code 4 Flashes: Primary or Auxiliary Limit Circuit Open
What Does Code 4 Flashes Mean?
Four flashes on the GMSS960803BN's Integrated Control Module indicate the primary or auxiliary limit circuit is open. The limit switch is a temperature safety that opens when the air moving across the heat exchanger gets too hot, shutting off the burners to protect the heat exchanger from overheating. When it trips, the circulator blower keeps running to pull heat away, and the switch resets itself once temperatures fall back into a safe range.
Because the GMSS960803BN is a single-stage furnace, it always fires at one fixed heat output — there is no low stage to fall back on — so the amount of air moving across the heat exchanger is what keeps temperatures in check. Anything that chokes that airflow lets heat pile up until the limit opens. By far the most common cause is a dirty or blocked air filter; Goodman also lists restrictive ductwork, closed or blocked registers, improper blower speed, a failing circulator blower, or loose wiring.
This is one of two heat-safety limits on the board, and it helps to tell them apart. The 4-flash primary/auxiliary limit responds to general overheating from poor airflow. The 6-flash rollout limit is a different and more serious device that reacts to flames escaping the burner area, usually from a venting problem — so if you also see six flashes, treat that as the higher priority.
What You'll Notice
- The blower runs continuously, often blowing cool or room-temperature air, while the burners stay off
- The furnace heats for a while, shuts the burners off, then restarts once it cools (short cycling on the limit)
- The air filter is visibly gray, dusty, or clogged
- Some rooms have closed, blocked, or covered supply registers
- The diagnostic LED blinks four times, pauses, and repeats
Common Causes
How This Is Diagnosed
The cause is isolated by working through the airflow path. The air filter is checked first because a clogged filter is the most common trigger, then supply registers and return grilles are confirmed open and unobstructed, and accessible ductwork is inspected for crushed or disconnected sections. If airflow is clearly adequate, attention moves to blower speed and blower performance, and finally to the limit switch itself and its wiring. The filter and register checks are homeowner-safe; blower and limit-switch work are not.
How to Fix It: Replace the Air Filter and Restore Airflow
What You'll Need
- Replacement air filter (matching size) 🛒 Find at FiltersFast · 🛒 Find at Amazon
- Flashlight
Steps
- Turn off power at the breaker and shut off the gas supply Flip the furnace circuit breaker to OFF and turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas, leave immediately and call your gas company. Allow the furnace to cool before working near it.
- Replace the air filter Locate the air filter in the blower compartment or return-air duct. Remove the old filter — if it is gray, dusty, or visibly clogged, it is likely the cause. Install a new filter of the correct size with the airflow arrow pointing toward the blower.
- Open all supply registers and return grilles Walk through every room and make sure all supply registers and return-air grilles are fully open and not blocked by furniture, rugs, curtains, or other items. Restricting too many registers starves the furnace of airflow.
- Inspect accessible ductwork Check visible ductwork in the basement, attic, or crawl space for collapsed flex duct, disconnected sections, or crushed portions that restrict airflow.
- Restore power and gas, then test Turn the gas supply valve to ON (handle parallel to the pipe) and flip the circuit breaker to ON. Set the thermostat to call for heat and monitor the furnace through a complete cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- The code returns after replacing the filter and opening all vents
- The blower motor sounds labored or is not spinning at full speed
- The limit trips within a minute or two of the furnace starting
- You suspect the ductwork is undersized or badly restricted
- You also see six flashes (rollout limit circuit open)
Frequently Asked Questions
Why does my Goodman GMSS960803BN blower keep running with cold air?
When the 4-flash limit trips, the board keeps the circulator blower running to cool the heat exchanger while the burners stay off, so you feel cool air. Clearing the airflow restriction — usually a dirty filter — lets the burners fire again.
How often should I change the filter to avoid this code?
It varies with filter type, pets, and dust levels, but many homeowners replace a 1-inch filter every one to three months. If the 4-flash code appears, check the filter first regardless of how recently it was changed.
Can a dirty filter really shut the whole furnace down?
Yes. A clogged filter starves the heat exchanger of airflow, heat builds up, and the limit switch opens to protect the furnace. It is the single most common cause of this code on single-stage furnaces like this one.
The filter is clean but I still get 4 flashes — now what?
If airflow through a clean filter and open registers is fine, the likely causes shift to restrictive ductwork, low blower speed, a weak blower motor, or a failing limit switch. Those require a technician to measure airflow and test the components.
✓ Verified against manufacturer service manual — March 2026