Lennox Furnace Error Code LED#1 Alternating Fast Flash / LED#2 Alternating Fast Flash

This alternating fast flash pattern on both LEDs indicates the control board detected a fault specifically during the ignitor warm-up period. The code points to one of three conditions: an improper main ground, a broken or open ignitor circuit, or line voltage that has dropped below 75 volts. Because the fault window is limited to the warm-up period, the problem is closely tied to the ignitor circuit or the electrical supply powering it.

A reset may temporarily clear the fault and allow the furnace to attempt another ignition cycle, but the code will return if the underlying problem is not corrected. If the ignitor is broken, the ground is faulty, or the supply voltage is insufficient, the control board will detect the same condition on the next warm-up attempt and generate the fault again. Resetting without diagnosing the root cause is not a repair.

Repair costs depend on the root cause. If the fix is simply tightening a ground wire or a loose connector, you may only pay a diagnostic service call fee, typically in the range of $75 to $150. A replacement hot surface ignitor, the most common hardware fix, usually runs $30 to $80 for the part, plus labor. If the issue involves a low-voltage transformer or a control board, costs can rise to the $200 to $600 range depending on parts and labor rates in your area.

Start by verifying the furnace has a solid ground connection at the chassis and at the electrical panel. Next, disconnect the ignitor and measure its resistance with a multimeter: it should read between 10.9 and 19.7 ohms. If the resistance is out of range or reads open circuit, replace the ignitor. Also measure line voltage at the furnace terminals during a start attempt to confirm it stays above 75 volts AC.

The furnace will not complete ignition while this fault is active, so there is no immediate combustion hazard from the code itself. However, a low line voltage condition or a grounding fault can pose broader electrical safety concerns and should not be ignored. It is best to have the system diagnosed and repaired before continuing regular operation.