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.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
Because the second-stage relay lives on the integrated control, a persistent 206 prices as a board replacement at $400–900 installed — but this is the rare board-level fault where the timing of the spend is yours to choose, and choosing well is worth real money. The furnace keeps heating on first stage for the rest of the demand, which means no emergency: you can book a normal-hours visit instead of paying the repair guide's $100–200 after-hours premium, and you have time to get a second quote on the board, which the guide recommends for anything over ~$500. The one way this alert gets expensive is being ignored until the first hard cold snap turns a Service Soon into an emergency call at emergency rates.
DIY or call a pro
There is nothing to attempt at home — the faulty relay is a soldered component on the main control — but there is something to verify, and it is worth doing before paying for anything: confirm the symptom matches the alert. A furnace stuck on first stage feels like longer, gentler run times and a house that holds temperature in mild weather but falls behind in cold; if instead the furnace is short-cycling or not heating at all, something beyond this alert is going on and the visit should be sooner. Mention to the technician whether high fire has ever worked since the alert appeared — an intermittent second stage points the diagnosis differently than one that never engages. Do not attempt to force high fire from the thermostat repeatedly; it re-tests a circuit already known faulty and proves nothing the fault history has not already recorded.
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.