Trane TUD1B080A9H31B Error Code 4 Flashes: Open Temperature Limit Circuit

What Does Code 4 Flashes Mean?

The 50A65 IFC watches a high-temperature limit switch that opens when the air moving over the heat exchanger gets too hot. A 4-flash code means that limit opened and the board shut off the burners to protect the furnace, while the blower keeps running to carry the excess heat away and cool things down.

On this single-stage furnace, the heat output is fixed, so adequate airflow is what keeps temperatures in range. Almost all overheating comes from restricted airflow: a clogged air filter is the most common cause, followed by closed or blocked supply and return vents. Less often the blower motor is weakening and can no longer move enough air. Because insufficient airflow is also what drives heat-exchanger stress, this code is worth resolving promptly.

The homeowner-safe part of this fault is airflow — the filter and the vents. If the limit keeps opening after those are addressed, the cause is inside the furnace (blower motor, capacitor, or the limit switch itself) and belongs to a technician.

What You'll Notice

• The control LED flashes four times, pauses, and repeats.
• The burners shut off but the blower keeps running to cool the furnace.
• The furnace short-cycles — heating briefly, then shutting down on the limit.
• The air filter looks gray and clogged, or you cannot see light through it.
• Some rooms feel starved for air because supply or return vents are blocked.

Common Causes

How to Fix It: Check and Replace the Air Filter

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 circuit breaker to OFF and turn the gas shutoff valve 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 needs replacing.
3. Replace the air filter Insert a new filter of the same size, with the airflow arrow on the frame pointing toward the furnace (the direction air moves). Note the size printed on the old filter before discarding it.
4. Open and clear all supply and return vents Walk through your home and confirm every supply register and return grille is open and unobstructed. Move furniture, rugs, or curtains that block them — restricted airflow is what overheats the furnace.
5. Restore gas and power Turn the gas valve back to ON (parallel to the pipe), then set the breaker to ON. The limit switch resets automatically once the furnace cools, which may take several minutes.

When to Call a Professional

Contact a licensed HVAC technician if:

• The 4-flash code returns after replacing the filter and opening all vents.
• The blower does not run, or runs noticeably slower than usual.
• You hear grinding or squealing from the blower motor area.
• The furnace short-cycles even with a clean filter and clear airflow.
• A flame rollout switch (a separate switch with a manual reset button) has tripped.

✓ Verified against manufacturer service manual — March 2026