Lennox Furnace Error Code LED1 ALT FAST FLASH / LED2 ALT FAST FLASH

This alternating fast flash pattern from both LED indicators signals a fault tied to the ignitor circuit or a board-level issue. During ignitor warm-up, it indicates a broken ignitor, open ignitor circuit, bad main ground, or line voltage below 75 volts. When it appears at power-up, it points to a circuit board failure or incorrect control wiring. The timing of when the code appears is key to directing the diagnosis correctly.

No, you should not attempt to operate the furnace while this code is active. If the ignitor circuit is open or the line voltage is below the minimum threshold, the burner will not light reliably. Repeated failed ignition attempts can cause unburned gas to accumulate, which is a safety hazard. Address the underlying fault before restoring normal furnace operation.

Repair costs depend heavily on the root cause. Ignitor replacement is typically the least expensive fix, with parts generally ranging from roughly $20 to $80 and labor adding to that total depending on your market. A failed control board is the most expensive scenario, with board costs typically ranging from $150 to $400 or more before labor. If the fault is simply a loose ground wire or a low-voltage supply issue from the electrical panel, the repair cost may be limited to a service call fee.

Start by measuring the ignitor resistance with a multimeter. A healthy ignitor reads between 10.9 and 19.7 ohms. If the reading is outside that range or shows an open circuit, the ignitor needs replacement. Also check that the main ground connection is secure and that line voltage at the furnace is above 75 volts. If the code appears only at power-up, inspect the control board wiring before suspecting board failure.

A power cycle may temporarily clear the fault display, but if the underlying problem is not corrected the code will return as soon as the furnace attempts another ignition cycle. Resetting the unit without repairing the ignitor, ground connection, or voltage issue will not produce a lasting fix. If the fault appears immediately at power-up after a reset, the control board wiring or the board itself requires attention.