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

This alternating flash pattern on both diagnostic LEDs indicates a Watchguard lockout caused by the burners failing to ignite. The furnace attempted to light the burners one or more times but could not confirm a flame, so the control board shut down the system as a safety precaution. The furnace will remain locked out until the ignition problem is identified and corrected.

The furnace is already locked out and will not produce heat or release gas while this code is active, so there is no immediate danger from continued failed ignition attempts. However, you should not attempt to bypass or override the Watchguard lockout. The lockout exists to prevent unburned gas accumulation, and the root cause should be diagnosed and repaired before the system is returned to normal operation.

Repair costs vary depending on the failed component. Flame sensor cleaning or replacement is typically the least expensive fix, often falling in the range of $75 to $200 including labor. A hot surface ignitor replacement generally runs $150 to $350. Gas valve replacement is more involved and can range from $300 to $600 or more depending on the part and local labor rates. A service call for diagnosis alone usually costs $80 to $150 before any parts.

Start by confirming the gas supply is on and that other gas appliances are working. Then inspect the flame sensor for buildup and clean it if needed. Check the hot surface ignitor for visible cracks and test the gas valve wiring for continuity. If basic checks do not resolve the issue, a qualified technician should measure gas pressure and test the ignition system components under live conditions.

A manual reset by cycling power to the furnace may clear the lockout temporarily and allow one more ignition sequence to run. However, if the underlying cause is not corrected, the furnace will fail to ignite again and return to lockout. Resetting without repairing the problem is not a long-term solution and repeated lockout cycles can stress system components.