Lennox furnace Alert code 205: Gas valve relay contact closed
Alert 205 means the gas valve relay contact is closed when it should not be. Lennox rates it Service Urgent and says to replace the furnace control if the issue continues.
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 relay on the furnace control is what energises the gas valve. This alert means the control has detected that relay's contact closed at a time it was not commanded to be. That is a specific and serious finding: a relay stuck closed can energise the gas valve outside a controlled sequence. Lennox's guidance is direct — check wiring on the control and the gas valve, note that the relay is located on the furnace control, and if the issue continues, replace the furnace control. That last instruction is unusually decisive and reflects that a stuck valve relay is not something to live with.
Most common causes
- A failed relay on the furnace control, welded or stuck closed.
- Wiring fault between the control and the gas valve.
- A control board fault causing the relay to be misread or misdriven.
- Electrical damage to the control from a surge.
- A control approaching the end of its service life.
How to diagnose it
- Note the alert number precisely. Lennox distinguishes 204, 205 and 206 as different gas valve faults with different remedies.
- Do not attempt to inspect the control or the gas valve wiring.
- Note any smell of gas and act on it immediately.
- Note whether the alert has appeared before, since Lennox's replace-the-control instruction is conditioned on the issue continuing.
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.
- With no gas smell, turn the furnace off and call a licensed technician.
- Lennox's documented remedy is to check wiring on the control and gas valve, and to replace the furnace control if the issue continues.
- Do not reset repeatedly hoping it clears. A relay that can stick closed can energise the gas valve when it should not.
Notes
A stuck valve relay is one of the few furnace faults where the manufacturer gives an explicit replace-the-board instruction. Control boards are otherwise hard to condemn confidently, so that guidance is worth relying on rather than second-guessing.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
Lennox's endgame for a persistent 205 is stated in its own guidance — replace the furnace control — which prices this alert as a control board job at $400–900 installed, with communicating OEM boards toward the top of that band. The wiring check comes first and occasionally saves the board: a short on the valve circuit can mimic a welded relay for the cost of a diagnostic visit ($75–150) plus a wiring repair. This is also one board replacement not worth second-guessing on price for long. The usual advice to slow down and re-quote a $700 board assumes the furnace is safe to run while you shop; a control that cannot guarantee its gas valve relay is open is not that furnace.
DIY or call a pro
No homeowner path exists here, and the specific reason deserves stating: the stuck component switches the gas valve, so "seeing if it does it again" means giving a faulty control repeated authority over fuel. Shut the system down at the thermostat — and if you have any reason to think the burners have run without a heat call, switch the furnace off at its service switch too — then treat the call as urgent, matching Lennox's rating. Do not pull the blower door to look at the board; the relay's failure is electrical, not visible. The nearest cross-brand relatives show how seriously control families take this state: the shared Goodman/York board dedicates 10 red flashes to gas flow detected without a heat call, and York's version reads gas valve energised with no call for heat — same class of fault, same conclusion, the control can no longer be trusted to keep the valve closed.
What this looks like
From the outside, this usually shows up as the furnace turns over but never lights — that walkthrough covers what to check in order, and when it stops being a homeowner job.