York TM9V080B12MP11 Error Code 4 Red Flashes: Main Limit Switch Open
What Does Code 4 Red Flashes Mean?
A 4 Red Flashes code on your York TM9V080B12MP11 furnace indicates that the main limit switch has opened its normally closed contacts. This is a safety mechanism that shuts down the burner when the furnace's internal temperature gets too high, preventing damage to the heat exchanger and other components. When this happens, the control board turns on both the supply air blower and the inducer motor to help cool the furnace down.
The most common cause of this condition is a dirty air filter that restricts airflow through the furnace. When the blower cannot move enough air across the heat exchanger, heat builds up inside the furnace until the limit switch trips. Other possible causes include an improperly sized duct system, incorrect blower speed setting, incorrect firing rate, loose limit switch wiring, or a faulty blower motor.
If the main limit switch opens five times within a single call for heat, the control board will enter a one-hour soft lockout. After the lockout period, the furnace will automatically attempt to restart. If the underlying airflow issue is not resolved, the cycle will repeat.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty or clogged air filter restricting airflow | Most common | ✓ DIY fix → |
| Improperly sized duct system | Common | ✗ Call a pro → |
| Faulty blower motor | Common | ✗ 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 locate the gas supply valve (typically a yellow handle on the gas pipe near the furnace) and turn it 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
- The furnace enters the one-hour soft lockout repeatedly
- You hear unusual noises from the blower motor area
- The furnace short-cycles (turns on and off frequently without completing a heating cycle)
Sources
✓ Verified against manufacturer service manual — March 2026