Lennox furnace E275: Soft lockout — flame sensed out of sequence, signal now gone

E275 means the control detected flame when there should have been none, and has locked out for an hour. The flame signal has since gone, but the reason it was there has not been explained.

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

This is the stored counterpart to E241. E241 is the live condition, flame present when it should not be; E275 is the one-hour soft lockout recorded after the signal disappeared. That the signal is gone is not reassurance — it means the condition has stopped being visible, not that it has stopped existing. Lennox's instruction is the same either way: shut off the gas and check for a gas valve leak. The lockout clears when flame has been proven stable, which means a furnace with this in its history can be running normally by the time anyone looks at it, with the underlying valve fault untouched.

Most common causes

A gas valve leaking through when commanded closed.
A short in the flame sensing circuit reporting a flame that was not there.
A burner glowing hot enough after shutdown to generate a brief signal.
Damaged or shorted flame sensor wiring.
A failed control board misreading the sense input.

How to diagnose it

Do not dismiss this because the furnace is running now. The self-clearing behaviour is what makes it easy to ignore.
If there is any smell of gas, leave the building and call the gas utility from outside.
Use the code recall menu to check whether this has happened more than once. The control stores the last ten codes.
Everything else belongs to a licensed technician.

How to fix it

Shut off the gas supply to the furnace at the manual valve.
Call a licensed technician. This is not a homeowner repair.
Expect the gas valve to be checked for leak-through and the sense circuit checked for a short.
A furnace that has recorded this more than once should not be left running while it is investigated.

Notes

The code recall menu is genuinely useful here. A single stored E275 might be explained by a hot burner after shutdown; several of them is a pattern, and only the recall list shows that.

All codes for this manufacturer

Lennox furnace alert codes — every code, with what differs between models.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

Lennox ML193UHEK Installation Instructions 508565-01, May 2026 — Integrated Diagnostic Codes / Status of Equipment

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.