Lennox HVAC Error Code 4 Flashes

The 4 Flashes fault code on a Lennox furnace indicates that the high limit switch or the rollout switch has opened. Both are safety devices that shut down the burner when they detect excessive heat or flame rollout. The furnace will not resume normal operation until the cause is resolved and any tripped manual-reset switches are reset.

Yes, this code reflects a real safety condition. An open rollout switch in particular can indicate that combustion flames are escaping the burner compartment, which poses a fire and carbon monoxide risk. Do not repeatedly reset the furnace and run it without identifying the root cause, especially if the rollout switch is the device that tripped.

Resetting the furnace may temporarily restore operation if the high limit switch is an auto-reset type and the underlying condition has resolved, such as after a filter change. However, if the rollout switch tripped, it is often a manual-reset device that requires physical access to reset at the switch itself. If the root cause is not corrected, the fault will return and the furnace will shut down again.

Some causes of the 4 Flashes code are DIY-friendly, such as replacing a dirty air filter, opening closed vents, or clearing a blocked return air grille. However, if the rollout switch has tripped, has a manual reset button that needs to be pressed, or if the problem keeps recurring, a qualified HVAC technician should inspect the system for more serious issues like a cracked heat exchanger or flue obstruction.

Repair costs vary widely depending on the cause. Replacing an air filter costs very little, while a blower motor replacement typically runs in the range of $300 to $600 including labor. If a cracked heat exchanger is involved, costs can range from $500 to well over $1,500, and in older units replacement of the entire furnace may be more cost-effective.