Lennox furnace Alert code 270: Flame failed to ignite — Watchguard
Alert 270 means the furnace is in Watchguard mode because the igniter could not light the flame. Lennox describes it as a five strike condition during a single demand.
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
Watchguard is Lennox's term for a soft lockout, and this alert means the furnace entered it after five failed ignition attempts within one heating demand. The five strike framing is useful: this is not one failure but a documented count, and the furnace stopped deliberately rather than continuing to deliver gas to a burner that would not light it. Lennox's checklist covers the expected ground — gas flow, whether the igniter is lighting the burner, flame sensor current — and adds two that are often the actual cause: dirty filters, and a blocked cold end heater box or condensate drain with cracked hoses.
Most common causes
- Insufficient gas flow, which Lennox lists first.
- The igniter not lighting the burner.
- Low flame sensor current, so a real flame goes undetected.
- Dirty filters, which Lennox names explicitly.
- A blocked cold end heater box or condensate drain.
- Cracked hoses in the pressure switch circuit.
- A failed hot surface igniter.
How to diagnose it
- Check whether other gas appliances work, which separates a gas supply problem from a furnace problem.
- Check the furnace's manual gas valve is open, and on propane that the tank has fuel.
- Check the air filter, since Lennox lists dirty filters among the causes of this ignition fault.
- Check the condensate drain, since Lennox names it here too.
- Note whether the furnace produces heat occasionally, which would indicate Watchguard releasing and the fault recurring.
How to fix it
- Restore the gas supply if that is the cause.
- Replace the air filter and clear a blocked condensate drain — both are homeowner work and both are on Lennox's list for this alert.
- Igniter replacement, flame sensor cleaning, and gas pressure work are technician tasks.
- Do not reset repeatedly to force a run. Each attempt delivers gas to a burner that may not light it.
Notes
It is worth noticing that Lennox puts filters and condensate drains on the list for an ignition failure. Those are not obvious causes of a furnace failing to light, and both are things a homeowner can check before calling.