York TM9T100C16MP11 Error Code 4 Red Flashes: Main Limit Switch Open

What Does Code 4 Red Flashes Mean?

A 4 red flashes code on the York TM9T100C16MP11 means the main limit switch has opened its normally closed contacts. This limit is a safety device that shuts off the burner when the temperature inside the furnace rises too high, protecting the heat exchanger. When it trips, the board runs the supply air blower and inducer to cool the furnace down.

The most common cause is a dirty air filter restricting airflow. When the blower cannot move enough air across the heat exchanger, heat builds up until the limit opens. Other causes include an improperly sized duct system, an incorrect blower speed setting, an incorrect firing rate, loose limit-switch wiring, or a faulty blower motor.

This code sits at the start of an escalation chain on this board. If the limit stays open longer than five minutes, the control assumes the blower is not running and hard-locks with 11 red flashes; if it stays open beyond fifteen minutes, it assumes a rollout switch has opened and shows 5 red flashes. And if the main limit opens five times within a single call for heat, the board enters a one-hour soft lockout. Restoring airflow early prevents these more serious lockouts.

What You'll Notice

• The furnace blows cool or room-temperature air, then the burner shuts off.
• The furnace short-cycles — starting and stopping frequently without completing a full cycle.
• The blower keeps running after the burner stops as the furnace cools.
• The air filter is visibly dirty or clogged when held up to a light.
• Some rooms stay cool and airflow at the registers feels weak.

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 Locate the circuit breaker for your furnace and flip it to OFF. Locate the gas supply valve (typically a 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.
2. Locate and inspect the air filter The filter is usually in the return air duct near the furnace or in a slot on the side or bottom of the furnace cabinet. Slide it out and hold it up to a light. If you cannot see light through it, it is clogged and needs replacement.
3. 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.
4. Check all supply and return vents Walk through your home and make sure all supply registers and return grilles are open and unobstructed. Move any furniture, rugs, or curtains blocking them, since closed or blocked vents restrict airflow and cause overheating.
5. Restore gas supply and electrical power Turn the gas supply valve back to ON (parallel to the pipe), then flip the furnace breaker back 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 code returns after replacing the filter and confirming 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
• The code escalates to 11 red flashes (blower failure) or 5 red flashes (rollout)
• You hear unusual noises from the blower motor area or the furnace short-cycles constantly

✓ Verified against manufacturer service manual — March 2026