Amana AMSS920803BN Error Code 4 Flashes: Primary or Auxiliary Limit Circuit Open

What Does Code 4 Flashes Mean?

Four flashes on the Amana AMSS920803BN indicate the primary or auxiliary limit circuit is open. The furnace got too hot, and the high-temperature limit switch tripped to protect the heat exchanger by shutting off the burners. The circulator blower keeps running to pull heat out of the furnace and bring the temperature back down.

The limit switch only trips when there is not enough airflow over the heat exchanger to carry heat away. On this single-stage furnace the blower runs at a fixed heating speed, so anything that chokes airflow — most commonly a clogged filter — pushes internal temperatures past the safe threshold quickly.

By far the most common cause is a dirty or clogged air filter, which a homeowner can check and replace. Other causes include blocked or restrictive ductwork, closed supply or return vents, an uncommon faulty limit switch, or a failing circulator blower motor. This code is related to six flashes (code-6, rollout limit) in that both involve safety limits, but four flashes is an overheat from restricted airflow, while a rollout means combustion is actually escaping the burner box — a more serious condition.

What You'll Notice

• The blower fan runs continuously, even between cycles, and the air feels cool or lukewarm
• The single diagnostic LED blinks four times, pauses, and repeats
• The furnace produces little or no heat while the fan keeps running
• The air filter looks gray and clogged when you pull it out
• Some rooms feel starved of airflow, or many supply vents are closed

Common Causes

How This Is Diagnosed

The cause is isolated by working through the airflow path. Check the filter first, then confirm supply and return vents and dampers are open, then look for restrictive or collapsed ductwork. If airflow is clearly adequate and the code still trips, a technician tests the limit switch and the blower motor speed and performance before condemning either part.

How to Fix It: Replace the Air Filter and Restore Airflow

What You'll Need

• Replacement air filter (correct size for your furnace) 🛒 Find at FiltersFast · 🛒 Find at Amazon
• Flashlight

Steps

1. Turn off power at the breaker and shut off the gas supply Turn the furnace circuit breaker to OFF and turn the gas shutoff valve to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave immediately and call your gas company from outside the home.
2. Locate and inspect the air filter Find the filter in the slot between the return duct and the blower compartment, or inside the blower compartment itself. Slide it out and hold it up to a light. If you cannot see light through it, it is too dirty and needs replacing.
3. Install a new filter of the correct size Fit a new filter of the same dimensions, with the airflow arrow on the frame pointing toward the furnace (away from the return duct). Slide it fully into the slot.
4. Open and clear supply and return vents Walk the home and make sure supply registers and return grilles are open and unobstructed by furniture, rugs, or drapes. Avoid closing off more than about 20 percent of the supply vents, since that restricts airflow and can retrip the limit.
5. Restore power and gas, then test Turn the gas valve ON and switch the breaker ON. Let the furnace cool for several minutes, then set the thermostat to call for heat and watch it run a complete cycle.

When to Call a Professional

Contact a licensed HVAC technician if:

• The limit trips again after replacing the filter and opening all vents
• The blower motor does not run or makes grinding or squealing noises
• Airflow at the supply vents is very weak even with a clean filter
• The furnace overheats and shuts down on nearly every heating cycle
• You suspect undersized, disconnected, or collapsed ductwork

✓ Verified against manufacturer service manual — March 2026