Lennox Furnace Error Code DIAG#1: On / DIAG#2: Flash

This code means the primary or secondary limit switch has opened because it detected temperatures above a safe threshold inside the furnace. The limit switch is a thermal safety device designed to shut down the gas valve or draft motor before the unit can overheat. Because the switch is an auto-reset type, it will restore continuity once the furnace cools down, and the unit may start again on its own. However, repeated tripping indicates an underlying condition such as poor airflow or a failing component that requires diagnosis.

Running the furnace while this code is active and recurring is not advisable. The limit switch is activating because internal temperatures are exceeding safe operating limits. Continued operation without correcting the cause can stress the heat exchanger, potentially leading to cracks that allow combustion gases to enter the living space. Start by checking the air filter and airflow before running additional heating cycles.

Allow the furnace to cool completely, then use a multimeter to check continuity across each limit switch. A properly functioning auto-reset switch should show continuity when the furnace is cold. If a switch shows no continuity while cold, the switch has failed and needs to be replaced. If the switch reads continuous when cold but the fault code returns after the furnace runs for a few minutes, the switch is operating correctly and the real problem is an airflow restriction, blower issue, or other source of overheating that must be traced and corrected.

The limit switch in this fault is an auto-reset type, meaning it will close on its own as the internal temperature drops back to a safe level. You may not need to manually reset the furnace control board at all. However, if the root cause of the overheating is not corrected, the switch will continue to trip every heating cycle, and eventually the furnace may escalate to a hard lockout requiring a manual reset or service call.

Repair costs vary widely depending on the root cause. Replacing a clogged air filter costs only a few dollars and is a DIY task. A failed blower capacitor typically runs in the range of $80 to $200 including labor. Replacing a faulty limit switch itself generally falls in the range of $100 to $300 with labor. If the heat exchanger is cracked and needs replacement, costs can climb significantly, often into the range of $500 to $1,500 or more depending on the furnace model.