Trane TUD1B080A9H31B Error Code 4 Flashes: Open Temperature Limit Circuit

What Does Code 4 Flashes Mean?

A 4-flash code on the Trane White-Rodgers 50A65 board means the high-temperature limit switch or flame rollout switch has opened. These are safety devices that shut down the burners when the heat exchanger temperature exceeds safe limits. The furnace is overheating.

The most common reason a furnace overheats is restricted airflow. When the air filter is clogged with dust and debris, or when supply and return vents throughout the house are closed or blocked by furniture, the blower cannot move enough air across the heat exchanger. Heat builds up inside the furnace instead of being distributed through the ductwork. When the heat exchanger reaches a dangerous temperature, the limit switch opens and shuts everything down.

This is one of the most common furnace problems and often has the simplest fix — replacing a dirty air filter and making sure vents are open. However, if the filter and vents are fine, the problem may be a failing blower motor or dirty blower wheel, which requires professional service.

How to Fix It: Check and Replace the Air Filter + Open All Vents

What You'll Need

• New air filter (same size as current filter — check the dimensions printed on the edge)
• Flashlight (optional, for inspecting vents)

Steps

1. Turn off power AND shut off gas supply Switch the furnace power off. Turn the gas shutoff valve to OFF (perpendicular to the pipe). Wait 5 minutes. If you smell gas at any point, leave the area immediately and call your gas company.
2. Locate and remove the air filter On most Trane furnaces, the air filter is located at the bottom of the furnace where the return air duct connects, or in a slot between the return duct and the blower compartment. Slide the existing filter out. Note the size printed on the filter frame (e.g., 16x25x1, 20x25x4).
3. Inspect the old filter Hold the filter up to a light. If you cannot see light through it, or if it is visibly gray/dark with dust and debris, it is overdue for replacement. Even if the filter does not look terrible, replace it if it has been in service for more than 90 days (standard 1-inch filters) or 6 months (4-inch media filters).
4. Install the new filter Slide the new filter into the same slot, making sure the airflow arrow printed on the filter frame points TOWARD the furnace (in the direction of airflow). The arrow should point toward the blower, not away from it.
5. Check all supply and return vents throughout the house Walk through every room and verify that all supply registers (where warm air comes out) and return air grilles (where air is pulled back to the furnace) are fully open and not blocked by furniture, rugs, curtains, or other objects. Open any closed registers. A rule of thumb: never close more than 20% of your supply registers, even in unused rooms.
6. Restore power and gas, then test Replace the furnace access panel if you opened it. Turn the gas supply back ON (parallel to the pipe). Restore furnace power. Set the thermostat to call for heat. The limit switch should automatically reset once the furnace cools down (this may take 5-10 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 motor sounds unusually loud, squeals, or does not run at all
• The limit switch keeps tripping every heating cycle
• You notice a burning smell from the furnace
• The furnace is more than 15 years old and has never had the blower wheel cleaned