Lennox furnace Alert code 241: Flame out of sequence — still present
Alert 241 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. This condition is alert code 241. An earlier version of this page numbered it 250, which is the primary limit switch code; the two were transposed when the guide was first read.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
If the likeliest cause holds — a gas valve passing gas without a command — this is a $300–900 gas valve replacement, and the repair guide's note on that line (never a DIY part) is absolute here. Do not price-shop this one the way the guide normally encourages: the second-quote advice for repairs over $500 assumes the appliance can safely wait, and a furnace flagged for flame outside its ignition sequence cannot. Pay the diagnostic ($75–150, and after-hours premiums of $100–200 are justified spending for this alert), and if the valve is leaking, authorize the fix. The cheaper outcomes — a wiring fault holding voltage at the valve, or a flame-sensing anomaly — get found by the same visit.
DIY or call a pro
Alert 241's DIY scope is a single action, and it is the one Lennox itself puts first: shut off the gas. Use the appliance's gas shutoff valve, switch the system off, and get a professional out the same day — this is the alert where “it seems to be running fine now” means least, because the condition it describes is fuel flowing outside the control's sequence. Do not cycle the thermostat to test it, do not reset power to clear it, and if you smell gas at any point, leave the building and call the gas utility's emergency line before anyone else. Everything a technician will do — leak-checking the valve, measuring voltage at its terminals during and outside a call — happens on a live gas train, which is precisely why the homeowner portion of this alert is turning things off and making a phone call.
The same fault on other brands
Flame sensed when none should exist is among the most consistently coded faults in the industry — the flame sensor hub documents it across 11 control families. Carrier's code 22, Goodman's 5 flashes, and Trane's 5 flashes all describe this alert's exact condition, and the shared Goodman/York board splits out the valve-leak variant as 10 red flashes (gas flow detected with no call for heat). Lennox's distinguishing detail is the pairing: alert 241 while flame is still present, alert 275 once the signal has gone.
What this looks like
From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — that walkthrough covers what to check in order, and when it stops being a homeowner job.