Goodman GMSS960803BN Error Code 4 Flashes: Primary or Auxiliary Limit Circuit Open

What Does Code 4 Flashes Mean?

Four flashes on the Goodman GMSS960803BN diagnostic LED indicates the primary or auxiliary limit circuit is open. The limit switch is a safety device that shuts down the burners when the furnace's internal temperature exceeds a safe threshold, preventing damage to the heat exchanger.

When the limit switch trips, the blower runs continuously to cool the heat exchanger, but the burners will not fire. The limit switch resets automatically once the temperature drops to a safe level.

The most common cause is a dirty or blocked air filter, which restricts airflow across the heat exchanger and causes heat to build up. Other causes include restrictive ductwork, closed or blocked supply registers, a failing or underperforming blower motor, a faulty limit switch, or loose wiring connections.

Common Causes

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 Flip the furnace circuit breaker to OFF. Turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). Allow the furnace to cool before working near it.
2. 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.
3. 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.
4. 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.
5. 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 switch trips within minutes of the furnace starting
• You suspect the ductwork is undersized for the furnace
• You also see code 6 (rollout limit circuit open)

✓ Verified against manufacturer service manual — March 2026