Lennox furnace Alert code 250: Flame out of sequence — still present

Alert 250 means flame is sensed without a call for gas heating. Lennox rates it Service Urgent and its first instruction 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

This alert means the control is detecting flame at a time when no heating demand exists. Lennox's response ordering is what makes this page necessary: the first documented step is to shut off the gas, before checking for a gas valve leak or checking voltage to the gas valve. Manufacturers rarely lead a troubleshooting list with shutting off the fuel supply, and when they do it reflects that the most likely cause — a gas valve passing gas when it should be closed — is a condition that should not be allowed to continue while it is investigated.

Most common causes

A gas valve leaking through when it should be closed.
Voltage reaching the gas valve when it should not be, which Lennox has you check.
A stuck gas valve.
A short in the flame sensing circuit producing a false flame signal.
A control fault energising the valve incorrectly.

How to diagnose it

Note any smell of gas. If there is one, act on the fix below immediately rather than continuing.
Do not open the burner compartment while the furnace is powered.
Note that Lennox rates this Service Urgent and leads with shutting off gas — that ordering is the guidance, not a formality.
Note that the alert clears automatically when a heat call ends successfully, so it disappearing does not mean the valve was checked.

How to fix it

If you smell gas: leave the building, operate no electrical switches, and call your gas utility's emergency line or the fire service from outside. Do not return to investigate.
Lennox's first documented step is to shut off the gas. If you know where your furnace's manual gas shutoff is and can reach it safely without entering a space where you smell gas, closing it is appropriate.
Call a licensed technician. Lennox's remaining steps — checking for a valve leak, checking voltage to the valve, replacing the valve if needed — are all technician work.
Do not reset the furnace repeatedly. If the valve is passing gas, each cycle is another opportunity for it to accumulate.

Notes

Very few furnace troubleshooting lists begin with shutting off the fuel. Lennox putting that first on this alert is the clearest possible signal about how the fault should be treated.

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.