Lennox furnace E273: Soft lockout — recycles exceeded after flame failure
E273 means the furnace lit each time and then lost the flame signal partway through the run, repeatedly, until it locked out. This is the classic dirty flame sensor pattern.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.
What it means
The furnace lights, runs, and then the control stops reading flame current and shuts the gas off. It does that several times before locking out. Lennox's troubleshooting for this code is identical to its low flame current code: read the microamps using the control's flame signal mode, clean or replace the sensor, and measure the voltage from neutral to ground to confirm a good unit ground. That last step matters more than it looks. Flame rectification returns its few microamps through the burner and the chassis to the control, so a furnace with a marginal ground produces a weak flame signal with a perfectly clean sensor. Cleaning the sensor on that furnace works for a while and then does not.
Most common causes
- A flame sensor coated in oxide, which builds up over a heating season and is not visible as dirt.
- A poor chassis ground, or reversed line voltage polarity.
- A cracked or mispositioned sensor, so the flame does not fully envelop the rod.
- Burners partially blocked, so the flame does not reach the sensor consistently.
- A failing gas valve producing an unstable flame.
How to diagnose it
- Note whether the furnace runs for several seconds to a minute and then stops with the blower continuing — that is this pattern.
- Note when the flame sensor was last cleaned. This is routine maintenance rather than a defect.
- Ask for the flame current reading before and after cleaning. This control displays it directly, so there is no reason to guess.
- Note whether the fault returned quickly after a previous sensor cleaning, which points at ground rather than the sensor.
How to fix it
- Cleaning or replacing the flame sensor is the usual repair and is technician work in the burner compartment.
- Insist that the ground and line polarity be checked, particularly if a cleaning has already been tried.
- The lockout clears when a heat call finishes successfully.
Notes
E240 is the warning that the flame current is low while the furnace is still running. E273 is what happens if that is left alone. Seeing E240 in the code history before E273 is the furnace having told you in advance.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.