Lennox Furnace Error Code 4 Flashes

Four flashes on a Lennox furnace indicate that the primary limit switch has opened, triggering a burner shutdown to prevent overheating. The primary limit switch is a thermal safety device that trips when the temperature around the heat exchanger rises above its rated threshold. This fault is associated with control board 103085. The furnace will not restart normally until the root cause of the overheating is addressed.

Yes, the primary limit switch is a critical safety component designed to prevent heat exchanger overheating, which can lead to stress cracking over time. A cracked heat exchanger is a serious concern because it can allow combustion gases to mix with heated air distributed through the home. Do not disable or bypass the limit switch. Address the underlying cause of the fault before continuing to operate the furnace.

Repair cost depends on the specific cause. A replacement air filter typically costs between $5 and $30 and is a straightforward DIY task. Blower motor replacement generally runs between $200 and $600 including labor. A new primary limit switch is usually a lower-cost part, but total repair cost including a service call and labor typically falls in the $100 to $300 range for the switch alone.

Start by replacing the air filter if it has not been changed recently, then check that all supply and return vents throughout the home are fully open and unobstructed. After correcting any airflow issues, allow the furnace to cool for 15 to 30 minutes so the limit switch can reset. If the error returns after restart, the blower motor, blower wheel, or limit switch itself may need professional inspection and possible replacement.

A manual reset may temporarily clear the fault and allow the furnace to restart, but if the underlying cause of restricted airflow or a failed limit switch is not corrected, the error will return, often within the same heating cycle. Repeated limit trips without correction accelerate wear on the heat exchanger and blower components. A reset should only be used after identifying and fixing the root cause.