Trane TUH1B080A9H31A Error Code 4 Flashes: Open Limit Device
What Does Code 4 Flashes Mean?
A 4-flash code on your Trane TUH1B080A9H31A furnace indicates that the open high limit device has triggered. This is a safety device that shuts down the furnace when the internal temperature exceeds safe operating limits, protecting the heat exchanger and other components from damage.
The most common cause of this code is restricted airflow through the furnace, which prevents heat from being carried away from the heat exchanger fast enough. When air cannot flow freely, heat builds up inside the furnace cabinet until the limit switch trips. A dirty or clogged air filter is the most frequent culprit, but blocked or closed supply registers, a failing blower motor, or a blocked return air duct can also cause overheating.
If the limit switch continues to trip after the filter has been replaced and airflow obstructions are cleared, the problem may be more serious, such as a failing blower motor, a blower capacitor issue, or a heat exchanger problem that requires professional diagnosis.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty or clogged air filter | Most common | ✓ DIY fix → |
| Blocked supply or return air vents | Common | ✓ DIY fix → |
| Failed blower motor | Uncommon | ✗ Call a pro → |
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
- Turn off electrical power at the breaker and shut off the gas supply valve Locate the circuit breaker for your furnace in your electrical panel and flip it to OFF. Also turn the gas supply valve to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave your home immediately and call your gas company from outside.
- Locate and inspect the air filter The air filter is typically located in the return air duct near the furnace, or in a slot on the side or bottom of the furnace cabinet. Slide the filter out and hold it up to a light source. If you cannot see light through the filter, it is clogged and needs replacement.
- Replace the air filter Insert a new filter of the correct size, making sure the airflow arrow printed on the filter frame points toward the furnace (in the direction of airflow). Note the filter size printed on the old filter's frame before discarding it.
- Check all supply and return vents Walk through your home and make sure all supply registers and return air grilles are open and unobstructed. Move any furniture, rugs, or curtains that may be blocking vents. Closed or blocked vents restrict airflow and can cause the furnace to overheat.
- Restore gas supply and electrical power Turn the gas supply valve back to the ON position (parallel to the pipe), then flip the furnace circuit breaker back to ON. The limit switch should automatically reset once the furnace cools down, which may take several minutes.
When to Call a Professional
Contact a licensed HVAC technician if:
- The code returns after replacing the filter and ensuring all vents are open
- The blower fan does not come on or runs noticeably slower than usual
- You hear unusual noises from the blower motor area
- The furnace cycles on and off frequently (short cycling)
- The flame rollout switch has tripped (usually has a manual reset button on the furnace)
✓ Verified against manufacturer service manual — March 2026