What causes York Gas Furnace 4 red flashes?

A dirty or clogged air filter (the most common cause). Too many supply or return registers closed or blocked off. An undersized or restricted duct system. An incorrect indoor blower speed setting. An incorrect firing rate, for example manifold gas pressure set wrong. A weak or failing indoor blower motor, or a dirty blower wheel. Loose limit switch wiring.

How do I fix York Gas Furnace 4 red flashes?

Confirm the code: count the red flashes through the view port before turning off power, since a power reset clears the current flash. Four flashes, a 2-second pause, then repeating is code 4. York boards also store the last five codes; with no thermostat call active, press and release the LAST ERROR button to replay them. Check the air filter and replace it if it's dirty. This is the first thing York's manuals list. Walk the house and open any closed supply registers, and make sure return grilles aren't blocked by furniture, rugs or curtains. With the furnace running, check that the indoor blower actually comes on and pushes a strong flow of air out of the registers. Weak or no airflow points to the blower motor or a dirty blower wheel. If the furnace went into the one-hour soft lockout after fixing the filter, you can wait for it to retry on its own or turn the power off and back on to restart it. Never jumper or bypass the limit switch. York's manuals warn that bypassing any safety control lets the furnace operate under potentially hazardous conditions. Don't adjust gas pressure or try to repair the control board yourself; York says controls should be replaced, not repaired.

When should I call a pro for York Gas Furnace 4 red flashes?

Call a technician if 4 red flashes comes back with a clean filter and open registers, or if the code changes to 11 red flashes (limit open more than 5 minutes, usually a failed blower motor or blower wheel). A technician checks the blower motor and blower wheel, the blower speed setting, the temperature rise against the range on the furnace rating plate, the manifold gas pressure (firing rate), the limit switch and its wiring, and whether the duct system is sized correctly. Repeated overheating is hard on the heat exchanger, so a furnace that keeps tripping its limit is worth a professional check even if it seems to recover on its own.

Code: 4 red flashes🔥 York Gas Furnace

York Furnace 4 Red Flashes: Main Limit Switch Open (Meaning, Causes, Fixes)

Q: What does York Gas Furnace 4 red flashes mean?

Four red flashes on a York furnace control board mean the main limit switch has opened: the furnace got too hot inside, almost always because not enough air is moving across the heat exchanger. York's manuals say to check for a dirty filter, an undersized duct system, an incorrect blower speed setting, an incorrect firing rate, loose limit switch wiring or a faulty blower motor. The control keeps the blower and inducer running while the limit is open, and if the limit opens five times in one call for heat, it shows 4 red flashes and waits an hour before trying again. York (Johnson Controls) furnace boards show faults with an LED behind the view port in the blower door. Fault codes flash on and off the number of times equal to the code, with a 2-second pause between codes, so 4 red flashes is code 4. York's Y91E and TM9E installation manuals define 4 red flashes as: the main limit switch has opened its normally closed contacts. The limit switch is an automatic-reset high-temperature control on the vestibule panel near the gas valve; it protects the furnace from overheating caused by reduced airflow. While the limit is open, the control runs the supply air blower and inducer to cool things down, and the limit closes again on its own once the temperature drops. If the main limit opens five times within a single call for heat, the control shows 4 red flashes and enters a one-hour soft lockout, then tries again automatically. If the limit stays open for more than 5 minutes, York treats it as a blower that isn't working: the control goes into a hard lockout and flashes 11 red flashes instead, which needs a power reset after the problem is fixed. Four red flashes is different from 5 red flashes, which means the manual-reset rollout switch opened (or the limit stayed open longer than 15 minutes on the Y91E).

Reviewed 2026-10-23 · Based on 2 manufacturer & service sources

What the code means

York (Johnson Controls) furnace boards show faults with an LED behind the view port in the blower door. Fault codes flash on and off the number of times equal to the code, with a 2-second pause between codes, so 4 red flashes is code 4. York's Y91E and TM9E installation manuals define 4 red flashes as: the main limit switch has opened its normally closed contacts. The limit switch is an automatic-reset high-temperature control on the vestibule panel near the gas valve; it protects the furnace from overheating caused by reduced airflow. While the limit is open, the control runs the supply air blower and inducer to cool things down, and the limit closes again on its own once the temperature drops. If the main limit opens five times within a single call for heat, the control shows 4 red flashes and enters a one-hour soft lockout, then tries again automatically. If the limit stays open for more than 5 minutes, York treats it as a blower that isn't working: the control goes into a hard lockout and flashes 11 red flashes instead, which needs a power reset after the problem is fixed. Four red flashes is different from 5 red flashes, which means the manual-reset rollout switch opened (or the limit stayed open longer than 15 minutes on the Y91E).

Likely causes

A dirty or clogged air filter (the most common cause)
Too many supply or return registers closed or blocked off
An undersized or restricted duct system
An incorrect indoor blower speed setting
An incorrect firing rate, for example manifold gas pressure set wrong
A weak or failing indoor blower motor, or a dirty blower wheel
Loose limit switch wiring

Safe DIY checks

⚠ Before you start

Turn off power at the furnace/HVAC service switch and the breaker before opening panels. Shut the gas valve before any work near the burners, and never disconnect or alter the gas line. If you smell gas, leave the house and call your gas utility.

Confirm the code: count the red flashes through the view port before turning off power, since a power reset clears the current flash. Four flashes, a 2-second pause, then repeating is code 4. York boards also store the last five codes; with no thermostat call active, press and release the LAST ERROR button to replay them.
Check the air filter and replace it if it's dirty. This is the first thing York's manuals list.
Walk the house and open any closed supply registers, and make sure return grilles aren't blocked by furniture, rugs or curtains.
With the furnace running, check that the indoor blower actually comes on and pushes a strong flow of air out of the registers. Weak or no airflow points to the blower motor or a dirty blower wheel.
If the furnace went into the one-hour soft lockout after fixing the filter, you can wait for it to retry on its own or turn the power off and back on to restart it.
Never jumper or bypass the limit switch. York's manuals warn that bypassing any safety control lets the furnace operate under potentially hazardous conditions.
Don't adjust gas pressure or try to repair the control board yourself; York says controls should be replaced, not repaired.

🛠 When to call a pro

Call a technician if 4 red flashes comes back with a clean filter and open registers, or if the code changes to 11 red flashes (limit open more than 5 minutes, usually a failed blower motor or blower wheel).
A technician checks the blower motor and blower wheel, the blower speed setting, the temperature rise against the range on the furnace rating plate, the manifold gas pressure (firing rate), the limit switch and its wiring, and whether the duct system is sized correctly.
Repeated overheating is hard on the heat exchanger, so a furnace that keeps tripping its limit is worth a professional check even if it seems to recover on its own.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Furnace air filter

Match the size printed on your current filter's frame.

Search eBay →

⚙

Furnace limit switch (exact OEM part for your model)

Only after testing shows the switch itself is faulty; it usually trips because of an airflow problem.

Search eBay →

⚙

Furnace blower motor (exact OEM part for your model)

If the blower is weak or doesn't run. Model-specific; ECM motors often need a matching module.

Search eBay →

⚙

Carbon monoxide alarm

Recommended in any home with a gas furnace.

Search eBay →

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.

York Furnace 4 Red Flashes: Main Limit Switch Open (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Furnace air filter

Furnace limit switch (exact OEM part for your model)

Furnace blower motor (exact OEM part for your model)

Carbon monoxide alarm

Related codes

Sources