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

This indication means the furnace is powered on and operating normally. Both diagnostic LEDs flashing at the same time is a status signal from the control board, not an error code. An increased flash rate specifically indicates that a heating demand is active and the furnace is responding to a thermostat call for heat.

Yes, simultaneous flashing from both DIAG LEDs is a normal operational status and does not indicate any unsafe condition. The furnace is functioning as designed. If you have concerns about heating performance or unusual sounds or smells, those issues should be investigated independently of this indicator.

Resetting the furnace is not necessary and will not change this indication in a meaningful way. After a reset, the simultaneous flash will reappear once power is restored because it is the normal power-on status display. Resetting is only relevant when a true fault code is present, not during normal operation.

No repair is needed when both LEDs are flashing simultaneously. This pattern confirms normal operation and a healthy control board. If the furnace is not heating despite showing this pattern, the problem lies with another component such as the ignition system, gas valve, or a limit switch, and those components should be diagnosed separately.

Because this is a normal status indication and not a fault code, there is no repair cost associated with it. If a technician visit is needed to confirm normal operation or to investigate a separate heating issue found alongside this indication, diagnostic service call fees typically range from $75 to $150 depending on your location and service provider.