On the covered Lennox gas furnace, E125 means the control failed its self-check: an internal error or failed hardware, with the integrated control not communicating. It covers hardware errors such as flame-sense circuit faults and pin shorts. This is a critical alert and clears 300 seconds after the fault recovers. First, turn power off at the furnace switch, wait a minute and restore it once.
Try in this order
0 of 9 completedWhat to do first
ReversibleTurn power off at the furnace switch, wait a minute and restore it once.
Correct only the documented fault domain
ReversibleLennox says to cycle power on the control and replace it if the problem prevents service and is persistent. Control replacement is a technician repair.
Cycle the power once
ReversibleE125 is a control that failed its own self-check. Lennox's documentation says the control restarts if the error recovers; switch the furnace off at the breaker and on once.
Call a technician if it repeats
ReversibleA code that returns after a power cycle is a control board hardware problem.
Note the furnace's behavior before the alert
ReversibleWrite down whether the furnace was running, whether the blower or inducer was on and whether the display showed any other alert. E125 is a critical alert, so a record of what was happening when it appeared helps the technician decide whether the fault is a one-off or a hardware failure.
Cycle the power once
ReversibleSwitch the furnace off at its service switch, wait a full minute and switch it on. Lennox says to cycle power on the control. Do not repeat this over and over, because a failing control may need replacing and repeated restarts add no information.
Wait for the alert to clear
ReversibleThe alert clears 300 seconds after the fault recovers. If it shows again within a few minutes of starting a heat call, the fault is present and the control is probably failing its self-check on hardware, such as the flame-sense circuit.
Do not open the control compartment
ReversibleThe integrated control carries line voltage and 24 volts, and a flame-sense circuit fault can involve wiring that is hazardous to touch. Leave the panels on and the inspection to a technician, who can read the control's diagnostic history.
Call Lennox service with the control's details
ReversibleGive the model number of the furnace, the E125 code and what you saw. Lennox's guidance is to replace the control if the problem persists and prevents service, so ask the technician to confirm the fault with a test before the part is ordered.
Evidence ledger
Sources behind this answer
Exact questions
Common questions about “Lennox gas furnace code E125”
What does E125 mean on a Lennox gas furnace?+
The control failed its self-check: an internal error or failed hardware, with the integrated control not communicating. It covers hardware errors such as flame-sense circuit faults and pin shorts. This is a critical alert and clears 300 seconds after the fault recovers.
Is E125 an emergency on a Lennox furnace?+
No. It is an internal control check that can clear after a restart; call a technician if it repeats.
Is E125 a critical alert?+
Yes, Lennox lists it as a critical alert that clears 300 seconds after the fault recovers.
Can I replace the control myself?+
Control replacement is a technician repair.
Will cycling the power fix it?+
It may clear a one-off, but if the code returns the control is the likely cause.