Amana AMSS920803BN Error Code 4 Flashes: Primary or Auxiliary Limit Circuit Open
What Does Code 4 Flashes Mean?
Status code 4 (four LED flashes) on the Amana AMSS920803BN indicates that the primary or auxiliary limit circuit is open. The furnace has overheated and the high-temperature safety switch (limit switch) has tripped to protect the heat exchanger. The circulator blower will continue running to cool the furnace down.
The limit switch is designed to shut off the gas burners when the temperature inside the furnace exceeds a safe level. This happens when there is not enough airflow over the heat exchanger to carry heat away. The blower fan keeps running to bring the temperature back down.
The most common cause is a dirty or clogged air filter that restricts airflow through the system. When the filter is blocked, not enough air passes over the heat exchanger, causing it to overheat. Other possible causes include blocked or restrictive ductwork, closed supply or return vents, a faulty limit switch, or a failed circulator blower motor.
Common Causes
How to Fix It: Replace the Air Filter and Check Airflow
What You'll Need
- Replacement air filter (correct size for your furnace) 🛒 Find at FiltersFast · 🛒 Find at Amazon
- Flashlight
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the gas shutoff valve to the OFF position (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
- Locate and inspect the air filter Find the air filter — it is typically in a slot between the return air duct and the furnace blower compartment, or inside the blower compartment itself. Slide the filter out and hold it up to a light. If you cannot see light through it, it is too dirty and needs to be replaced.
- Replace the air filter Install a new filter of the correct size, making sure the airflow direction arrow on the filter frame points toward the furnace (away from the return duct). Slide it fully into the slot.
- Check all supply and return vents Walk through your home and make sure all supply vents and return air grilles are open and unobstructed. Move furniture, rugs, or other items that may be blocking airflow. Close no more than 20% of supply vents.
- Restore power and gas, then test Turn the gas supply valve to ON and flip the circuit breaker to ON. Allow the furnace to cool down for several minutes. Set your thermostat to call for heat and monitor the furnace through a complete heating cycle.
When to Call a Professional
Contact a licensed HVAC technician if:
- The limit switch trips again after replacing the filter and opening all vents
- The blower motor is not running or makes unusual noises
- You notice very weak airflow from the supply vents even with a clean filter
- The furnace repeatedly overheats during every heating cycle
- You suspect the ductwork is undersized or has collapsed sections
✓ Verified against manufacturer service manual — March 2026