American Standard AUH1B080A9H31A Error Code 4 Flashes: Open Limit Device

What Does Code 4 Flashes Mean?

Code 4 (4 Flashes) on the White-Rodgers 50A65 Integrated Furnace Control (IFC) in the American Standard AUH1B080A9H31A means the high-temperature limit switch has opened. This safety switch trips when the air temperature around the heat exchanger climbs past a safe level, and the board shuts down the burners to protect the heat exchanger from heat damage.

On a furnace, overheating almost always comes down to restricted airflow on the supply side — not enough air is moving across the heat exchanger to carry the heat away, so temperature builds until the limit opens. A dirty or clogged air filter is by far the most common cause, followed by closed or blocked supply registers, a blocked return, or closed dampers. All of those are things a homeowner can safely check and correct.

It is worth distinguishing this from the 3-flash pressure switch error: the 4-flash limit fault is about the blower moving too little conditioned air across the heat exchanger, while the 3-flash fault is about combustion draft through the venting. If the filter is clean and every vent is open but the furnace still trips on the limit, the cause moves into professional territory — a weak or failing blower motor, a failed blower capacitor, a bad limit switch, or a heat-exchanger restriction.

What You'll Notice

• The furnace produces heat for a while, then shuts the burners off before the room reaches temperature.
• The blower may keep running (sometimes blowing cooler air) to shed heat after the burners cut out.
• The diagnostic LED shows a repeating group of 4 flashes.
• The furnace short cycles — heating and stopping in a repeating pattern.
• The air filter is visibly gray and clogged, or several vents are found closed or blocked.

Common Causes

How to Fix It: Check and Replace the Air Filter and Clear Airflow Restrictions

What You'll Need

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

Steps

1. Turn off electrical power at the breaker and shut off the gas supply valve Set the furnace breaker (or the furnace power switch) to OFF, and turn the gas supply valve to OFF (handle perpendicular to the pipe). If you smell gas at any point, leave your home immediately and call your gas company from outside.
2. Locate and inspect the air filter The filter sits in the return duct near the furnace or in a slot on the side or bottom of the cabinet. Slide it out and hold it up to a light — if you cannot see light through it, it is clogged and is very likely what caused the overheat.
3. Install a new filter of the correct size Note the size printed on the old filter's frame, then insert a new filter of the same size with the airflow arrow pointing toward the furnace (the direction air moves into the blower). Avoid a filter that is far more restrictive than the original, as an overly dense filter can itself starve airflow.
4. Open and unblock all supply and return vents Walk the house and confirm every supply register and return grille is open and clear. Move furniture, rugs, or curtains off vents. Closing off too many rooms starves the blower and can re-trip the limit.
5. Restore gas and power, then allow the furnace to cool Turn the gas valve back to ON (handle parallel to the pipe) and set the breaker/switch back to ON. The limit switch resets on its own once the furnace cools, which can take several minutes.

When to Call a Professional

Contact a licensed HVAC technician if:

• The 4-flash code returns after you replace the filter and confirm all vents are open
• The blower does not run, runs noticeably slower than usual, or makes grinding or squealing noises
• The furnace short cycles repeatedly even with clean airflow
• A flame rollout switch (a separate safety with a manual reset button) has tripped

✓ Verified against manufacturer service manual — March 2026