Lennox furnace Alert code 206: Gas valve second stage relay fault

Alert 206 means the second-stage gas valve relay is faulty. Lennox rates it Service Soon and notes the furnace will run on first stage for the rest of the heating demand.

Applies to: Lennox communicating two-stage 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 is a good illustration of graceful degradation. The second-stage relay controls high fire, and a fault in it means the furnace cannot step up. Rather than stopping, Lennox's control keeps running on first stage for the remainder of the heating demand — so the house still gets heat, just less of it. That is why Lennox rates this Service Soon rather than Service Urgent. The practical consequence is that the fault is easy to miss in mild weather and becomes obvious when it turns cold and the furnace cannot keep up.

Most common causes

A failed second-stage relay on the furnace control.
Wiring fault in the second-stage gas valve circuit.
A control board fault affecting only the second-stage output.
A gas valve fault specific to its second-stage solenoid.

How to diagnose it

Note whether the house heats slowly rather than not at all, which matches this fault.
Note whether the furnace ever reaches full output in cold weather.
Confirm the furnace is genuinely two-stage, since single-stage models have no second-stage relay.
Note the alert number, since Lennox's three gas valve alerts have different remedies.

How to fix it

Lennox's documented remedy is to replace the furnace control if it cannot operate second stage.
Control replacement is technician work.
Do not ignore this simply because the house is still warm. The furnace is running at reduced capacity and will be inadequate in a cold snap.
Book the repair before winter deepens rather than during it.

Notes

Lennox rating this Service Soon rather than Service Urgent is informative. The furnace is safe and functional at reduced output, which means this can be scheduled rather than treated as an emergency — a genuinely useful distinction that not every manufacturer makes explicit.

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.