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.
What the fix costs
E273 and E240 share one troubleshooting block in the 508565-01 table, and that identity is the pricing insight: this is the same weak-signal fault at a later, lockout-earning stage, so it carries the same repair — flame sensor cleaning at the repair guide's $80–200 — plus the urgency E240 lacked. The board's microamp readout keeps the transaction verifiable at both ends: a reading confirms the low signal before the cleaning and certifies the recovery after it, which for an $80–200 job is unusual accountability. Budget realism on the recurrence path: when a verified cleaning stops holding, the manual's neutral-to-ground voltage check is the next spend, and a grounding repair at roughly service-call cost is cheaper than the third and fourth cleanings it replaces.
DIY or call a pro
The pattern E273 records — lights, runs, loses the signal, relights, repeats to lockout — deserves one homeowner distinction before anyone opens the cabinet: is the flame actually going out, or only going unseen? Heat that noticeably stops and restarts mid-cycle suggests a real flame loss (draft or gas-side, a different investigation); steady heat punctuated by shutdowns suggests the sensing side, which is this code's home ground. You can make that call from the living room by what the supply registers do, and it is worth making out loud when you book. The hands-on work — microamp readings at the control, sensor cleaning at the burner, ground verification — sits on the flame-sensing circuit this site consistently assigns to professionals. Resist clearing the recall history meanwhile; a string of E273s beside a ground-fault code like E117 is a completed diagnosis waiting to be noticed.
What this looks like
From the outside, this usually shows up as the burner lights and then drops out a few seconds later — that walkthrough covers what to check in order, and when it stops being a homeowner job.
This can also show up as the pilot light will not stay lit — that page walks through thermocouple, draft and gas causes in order.