Lennox furnace Alert code 275: Flame out of sequence — Watchguard

Alert 275 means the furnace detected flame when there should be none, and has gone into Watchguard mode. Lennox's first instruction for it is to shut off the gas.

Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.

What it means

The control watches the flame sensor at times when the gas valve is closed and no flame should exist. If it reads a flame signal in that window, something is wrong in a way the furnace cannot safely work around, so it shuts down into Watchguard rather than continuing. The reason this matters more than most alerts is what it implies: either the gas valve is passing gas when it is commanded shut, or the flame sensing circuit is reporting a flame that is not there. The first is a gas leak into a hot chamber. Lennox does not treat this as a diagnostic exercise — the published instruction is to shut off the gas, check for a gas valve leak, and replace the valve if needed. Alert 250 reports the same underlying condition without the Watchguard lockout.

Most common causes

A gas valve leaking through when it is commanded closed, which is the possibility that dictates the response.
A short in the flame sensing circuit reporting a flame that is not present.
A burner still glowing hot enough after shutdown to produce a flame signal.
Damaged or shorted flame sensor wiring.
A failed control board misreading the sense input.

How to diagnose it

Do not treat this as something to investigate before acting. Lennox's own first step is to shut off the gas.
Note the alert number before clearing it, since 275 and 250 describe the same condition at different severities.
Note whether there is any smell of gas. If there is, leave the building and call the gas utility from outside.
Leave everything beyond that 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 under any circumstances.
Expect the technician to check the gas valve for leak-through and to check for voltage reaching the valve when it should be de-energised.
Expect the gas valve to be replaced if it is passing gas.

Notes

Watchguard is Lennox's term for a soft lockout the control clears itself, in this case on a successful ignition. That self-clearing is what makes the alert dangerous to ignore: the furnace may appear to recover and run normally while the underlying valve fault is unchanged.

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 Residential Communicating Systems Alert Code Guide, document 100017, March 2023 — Table 2, Alert Codes and Troubleshooting

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.