York TM9V080B12MP11 Error Code 4 Red Flashes: Main Limit Switch Open

What Does Code 4 Red Flashes Mean?

The main limit switch is a normally-closed safety that opens when the air temperature around the heat exchanger climbs too high. When it opens, this York board turns on the supply-air blower and inducer to pull heat out of the cabinet and protect the heat exchanger. Nearly always the root problem is airflow: not enough air is moving across the heat exchanger to carry the heat away, so temperature builds until the limit trips.

A dirty or clogged air filter is the most common airflow restriction and the one a homeowner can fix. Beyond that, the manual lists an improperly sized duct system, an incorrect blower-speed setting, an incorrect firing rate, loose limit-switch wiring, or a faulty blower motor — all of which are technician-level. On this variable-speed model an incorrect blower profile or a weak ECM motor can starve airflow even with a clean filter.

This code also sits at the center of an escalation ladder. If the limit stays open past five minutes, the board assumes the blower is not working and switches to an 11 Red Flashes hard lockout; if it stays open past fifteen minutes, the board assumes a manual-reset rollout switch has opened and switches to a 5 Red Flashes hard lockout. And if the limit opens five times within one call for heat, the board holds at 4 Red Flashes and enters a one-hour soft lockout.

What You'll Notice

• Warm air turns cool partway through a cycle, and the blower keeps running after the burners shut off.
• The furnace short-cycles — firing, overheating, shutting the burners, and repeating.
• The house struggles to reach the setpoint even though the furnace is trying to run.
• The filter is visibly gray and clogged, or supply/return registers are closed or blocked.

Common Causes

How This Is Diagnosed

Start with the airflow items a homeowner can address — the filter and the registers — because a restricted filter is the most common trigger. If overheating continues with a clean filter and open vents, a technician moves to the technician-level causes in the manual: measuring temperature rise against the rating plate, checking the blower-speed profile and firing rate, inspecting the ECM blower motor and wheel, and checking the limit-switch wiring. The escalation behavior (to 11 or 5 flashes) also helps them tell an airflow restriction apart from an actual blower failure or a rollout trip.

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 power at the breaker or service switch AND shut off the gas supply Flip the furnace circuit breaker (or the wall service switch near the furnace) to OFF, and turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). If you smell gas at any point, leave immediately and call your gas company from outside.
2. Locate and inspect the air filter The filter is usually in the return-air duct next to 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 little or no light passes through, it is clogged and needs replacing.
3. Replace the filter with the correct size Note the size printed on the old filter's frame, then insert a new filter of the same size with the airflow arrow pointing toward the furnace (the direction air moves). Avoid overly dense filters that your system was not designed for, as they can restrict airflow.
4. Open and clear all supply and return vents Walk the house and make sure every supply register and return grille is open and unobstructed by furniture, rugs, or curtains. Closed or blocked vents restrict airflow and can trip the limit switch.
5. Restore gas and power Turn the gas valve back to ON (handle parallel to the pipe), then switch the breaker or service switch back 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 even after a fresh filter is installed and all vents are open.
• The blower does not start, starts late, or clearly runs slower than normal.
• The furnace keeps entering a one-hour soft lockout, or escalates to an 11 or 5 red flash hard lockout.
• You hear unusual noises from the blower area, or the furnace short-cycles repeatedly.

✓ Verified against manufacturer service manual — March 2026