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

This fault code means the furnace control board has detected either a failure in its own internal self-check routine or a wiring error on the ignition or blower control circuit. Both diagnostic indicator LEDs staying continuously lit together is the board's way of flagging this specific condition. The furnace will not operate normally until the cause is identified and corrected. A power reset is the recommended first step in the diagnostic process.

No, the furnace should not be operated when this fault is active. The continuous illumination of both diagnostic LEDs indicates the control board cannot confirm its own circuit integrity or has detected an invalid wiring condition. Running the furnace in this state bypasses the safety logic the board is designed to enforce. Power should be turned off and the system should be inspected before attempting further operation.

Repair costs depend on the root cause. If the issue is a wiring correction only, you may pay only for a diagnostic service call, typically in the range of $75 to $150. If the control board requires replacement, parts and labor combined generally range from $300 to $600 or more depending on the specific model and local labor rates. Getting a diagnostic assessment first is the most cost-effective approach before authorizing any parts replacement.

Turning the main power off and back on is the first recommended diagnostic step and may clear the fault if it was caused by a transient power event. If the fault returns immediately after the reset, a wiring problem or a failed control board is the more likely cause. A reset alone will not fix an underlying hardware or wiring issue, so further inspection is necessary if the error reappears.

Start by turning the main power off, waiting a moment, and then restoring power to reset the control board. If the fault clears and does not return, the issue was likely a transient event. If the fault returns, inspect all wiring connections to the ignition control and blower control terminals and compare them to the unit's wiring diagram. If wiring checks out correctly and the fault persists, the control board itself may need to be replaced.