Lennox furnace Alert code 204: Check gas valve
Alert 204 means there is an issue with the furnace gas valve. Lennox rates it Service Urgent and directs a technician to check valve operation, wiring, and voltage to the valve.
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 monitors the gas valve and its circuit, and this alert means something about that relationship is wrong. Lennox's guidance names three checks — valve operation, valve wiring, and voltage reaching the valve — which between them cover whether the valve is being commanded correctly and whether it is responding. Because the gas valve is the component that admits fuel to the burners, a control that cannot verify its valve cannot safely run the furnace. There is no homeowner portion of this fault: every check Lennox lists is at a live gas control circuit.
Most common causes
- A failed gas valve.
- Wiring to the gas valve damaged, loose, or disconnected.
- No voltage reaching the gas valve from the control.
- A miswired gas valve, most often after recent service.
- A failed valve relay on the furnace control.
- Corroded connections in the valve circuit.
How to diagnose it
- Establish whether the furnace has been serviced recently, since a miswired valve does not develop on its own.
- Do not attempt to inspect the gas valve or its wiring — this is a live gas control circuit.
- Note any smell of gas and act on it immediately rather than continuing to diagnose.
- Note that Lennox says the alert clears automatically once the issue is corrected, so it disappearing means the condition changed, not necessarily that it was diagnosed.
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.
- This is entirely technician work — all three of Lennox's documented checks are at the gas valve.
- If the furnace was recently serviced, call whoever did the work.
Notes
Lennox splits gas valve problems across several alert codes rather than grouping them: 204 for a general valve issue, 205 for a relay contact stuck closed, and 206 for a second-stage relay fault. That granularity is useful to a technician and means the specific code number is worth reporting rather than just saying the furnace shows a gas valve fault.