Trane TUH1B080A9H31A Error Code 4 Flashes: Open Limit Device
What Does Code 4 Flashes Mean?
A 4-flash code on the Trane TUH1B080A9H31A means the open limit device tripped. This high-temperature safety switch shuts down the burners when the air moving over the heat exchanger gets too hot, protecting the heat exchanger from heat stress. On a fault, the White-Rodgers 50A65 Integrated Furnace Control (IFC) keeps the blower running to pull that heat out of the cabinet while it waits for the limit to cool and reset.
On this single-stage 80,000 BTU furnace, overheating almost always comes down to restricted indoor airflow: heat is being produced at full rate, but not enough return air is passing over the heat exchanger to carry it away. A dirty or clogged air filter is by far the most common cause, followed by closed or blocked supply registers and return grilles. Note the contrast with 3 flashes (Pressure Switch Error): that code is about the combustion venting side, while the 4-flash limit trip is about the indoor circulating-air side.
If the limit keeps tripping after the filter is replaced and vents are opened, the cause is deeper — a weak blower motor or capacitor moving too little air, an oversized/clogged duct restriction, or a limit switch itself failing — and those require professional diagnosis.
What You'll Notice
- The diagnostic LED repeats a four-flash pattern
- The blower keeps running (often for a long time) even though the burners are off
- The furnace short-cycles: burners light, then shut off on high temperature before the room warms up
- Airflow from the supply registers feels weak, or the filter is visibly gray and clogged
- Some rooms feel starved for air because registers or returns are closed or blocked
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 This Is Diagnosed
Start with airflow, cheapest first: inspect the filter and replace it if light does not pass through it, then walk the house to open every supply register and return grille. Those two checks resolve the large majority of limit trips. If the code returns with a clean filter and open vents, the next suspects are the blower motor and capacitor (too little air moved), a duct restriction, or a failing limit switch — all of which a technician measures and confirms.
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 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 filter is usually 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.
- Replace the air filter Insert a new filter of the correct size with the airflow arrow on the frame pointing toward the furnace. Note the size printed on the old filter's frame before discarding it.
- Open all supply and return vents Walk through your home and make sure every supply register and return grille is open and unobstructed. Move furniture, rugs, or curtains blocking them. Closed or blocked vents restrict airflow and cause the furnace to overheat.
- Restore gas supply and electrical power Turn the gas supply valve back to ON (parallel to the pipe), then flip the furnace breaker to ON. The limit switch resets automatically once the furnace cools, which can take several minutes.
When to Call a Professional
Contact a licensed HVAC technician if:
- The code returns after replacing the filter and opening all vents
- The blower does not come on, or runs noticeably slower than usual
- You hear unusual noises from the blower motor area
- The furnace short-cycles frequently even with good airflow
- The flame rollout switch has tripped (usually a small button with a manual reset on the burner box)
Frequently Asked Questions
What does 4 flashes mean on my Trane furnace?
It means the high-limit safety switch opened because the furnace overheated, usually from restricted airflow. Replacing a dirty filter and opening blocked vents is the most common fix.
Why does the blower keep running after the burners shut off?
That is intentional. When the limit trips, the control board runs the blower to remove built-up heat from the furnace and cool the heat exchanger until the limit resets.
How often should I change the filter to avoid this code?
It varies with filter type, pets, and dust, so check monthly and replace when it looks dirty. Many homes land somewhere around every one to three months, but conditions vary widely by region and household.
Sources
✓ Verified against manufacturer service manual — March 2026