Lennox Furnace Error Code LED1-ALT_FAST_FLASH/LED2-ALT_FAST_FLASH

This alternating fast flash pattern on both LEDs indicates that the furnace control board has detected one of three fault conditions: improper main ground, line voltage below 75 volts, or a problem with the ignitor or its circuit. The furnace will not attempt to start a heating cycle until the fault is resolved. Diagnosis involves checking the ground connection, measuring incoming line voltage, and testing ignitor resistance.

A power reset may temporarily clear the fault display, but the error will return on the next heating cycle if the underlying problem has not been corrected. Because this fault can involve a failed ignitor or low voltage condition, the furnace may lock out again quickly after a reset. It is best to diagnose and repair the root cause before relying on a reset.

Repair cost depends on the root cause. A replacement hot surface ignitor typically costs between $20 and $80 for the part, with labor adding $75 to $200 depending on your area and service provider. If the problem is a wiring issue or grounding connection, costs may be lower since no major parts are needed. A control board replacement, if required, can range from $150 to $400 or more for parts alone. A diagnostic service call is usually the first step to confirm what component is at fault.

Start by verifying the furnace is properly grounded and that line voltage at the unit measures above 75 volts. Next, disconnect the ignitor and measure its resistance with a multimeter; the correct cold resistance range is 10.9 to 19.7 ohms, and a reading outside this range or an open circuit means the ignitor needs replacement. Also inspect the ignitor wiring harness for loose or damaged connectors. If voltage and grounding are correct and the ignitor tests good, the control board relay may need further evaluation by a qualified technician.

The furnace will generally not operate in a heating cycle while this fault is active, which is a built-in safety function. You should not attempt to bypass the control board or force the furnace to run while this fault is present. Low voltage or grounding issues can pose electrical hazards, and a failed ignitor left unaddressed can lead to failed ignition attempts that allow gas to accumulate briefly. Have the system inspected promptly.