Goodman GMSS920803BN Error Code 4 Flashes: Primary or Auxiliary Limit Circuit Open

What Does Code 4 Flashes Mean?

Four flashes on the Goodman GMSS920803BN diagnostic LED indicate that the primary or auxiliary limit circuit is open. The limit switch is a safety device that shuts the burners down when the air temperature around the heat exchanger climbs above a safe threshold, protecting the heat exchanger from heat damage. When it trips, the Integrated Control Module runs the circulator blower continuously to pull heat out of the furnace, but it will not fire the burners.

On this single-stage furnace there is one fixed firing rate, so the heat exchanger always produces the same amount of heat during a call. That heat is only safe if enough air moves across it, which makes airflow the central issue behind this code. The most common cause is a dirty or blocked air filter starving the blower of return air. Restrictive or partially closed ductwork, blocked supply registers, an underperforming circulator blower, a faulty limit switch, or loose wiring can produce the same overheating trip. The limit resets on its own once the furnace cools.

This code is distinct from the 6-flash rollout condition on the same board. Four flashes is an over-temperature trip caused by too little airflow, and its leading cause is a homeowner-serviceable filter. Six flashes means the rollout limit sensed flames escaping the combustion chamber — a combustion and venting hazard that is never a DIY repair. If you ever see both, treat it as the more serious rollout condition.

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, then shuts the burners off and the LED shows four flashes
• The air filter is visibly gray, matted, or clogged when you pull it
• Airflow at the supply registers feels weak, or many registers are closed or blocked
• The problem tends to worsen over a heating season as the filter loads up with dust

Common Causes

How This Is Diagnosed

The cause is isolated airflow-first, because insufficient air across the heat exchanger is the usual trigger. A homeowner can safely start by checking the air filter and confirming that supply registers and return grilles throughout the home are open and unobstructed — a clogged filter is the single most common cause and is easy to rule out.

If airflow is restored and the code still returns, the remaining causes are service items: a technician measures the temperature rise across the furnace, checks the circulator blower speed and amperage, inspects the ductwork for restrictions or a closed damper, and tests the primary and auxiliary limit switches themselves for a switch that opens below its rated temperature.

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

1. Turn off electrical power and gas supply first Switch the furnace circuit breaker to OFF and turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). Let the furnace cool before working near it. If you smell gas, leave immediately and call your gas company from outside.
2. Replace the air filter Locate the filter in the blower compartment or return-air duct. Remove the old one — if it is gray, matted, or you cannot see light through it, it is very likely the cause. Install a new filter of the correct size with the airflow arrow pointing toward the blower.
3. Open all supply registers and return grilles Walk through every room and confirm supply registers and return grilles are fully open and not blocked by furniture, rugs, curtains, or boxes. Restricting return air is a common overlooked cause of overheating.
4. Inspect accessible ductwork Check the visible ducting in the basement, attic, or crawl space for a collapsed flex duct, a disconnected section, or a crushed run that would choke airflow. Do not open sealed duct sections — just look for obvious restrictions.
5. Restore power and gas, then test Turn the gas supply valve back to ON (handle parallel to the pipe) and switch the breaker to ON. Set the thermostat to call for heat and watch a full cycle to confirm the burners stay lit and the LED returns to steady.

When to Call a Professional

Contact a licensed HVAC technician if:

• The 4-flash code returns after you replace the filter and open all vents
• The blower motor sounds labored, cuts in and out, or does not seem to reach full speed
• The limit switch trips again within a few minutes of the furnace starting
• You suspect the ductwork is undersized or crushed and cannot inspect it safely
• You also see a 6-flash rollout code, which is a combustion hazard and takes priority

✓ Verified against manufacturer service manual — March 2026