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.

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.