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.
Related
- Alert code 207 — Hot surface igniter sensed open
- Alert code 273 — Flame failed in run mode — Watchguard
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
The suspects behind five failed lights sort themselves by price. Most common and cheapest to fix: a worn hot surface igniter, which the ignitor cost guide prices at $150–350 installed (part alone $20–60 universal, $40–120 OEM) — igniters age on a three-to-seven-season clock, so a Watchguard 270 on an older furnace is often just the clock expiring. Next: a flame sensor too dirty to prove the flame that did light, at $80–200 for a cleaning. The expensive tail is gas-side — a valve problem at $300–900, never DIY. Lennox listing dirty filters and a blocked condensate path among the causes is the reminder that some 270s cost nothing but maintenance. More codes like this across brands: igniter fault codes.
DIY or call a pro
Two things are legitimately yours, and both are on Lennox's own cause list: a genuinely dirty filter, and a look at whether condensate is backing up near the furnace. Fix those if found, and allow the furnace its next attempt when the Watchguard period ends. The discipline this alert demands is about resets: five strikes already happened, and each manual power-cycle to force another round delivers gas to a burner that has repeatedly failed to light it. One retry after correcting something visible is reasonable; serial resets to squeeze out heat are how a soft lockout becomes a service call with a story attached. Everything at the burner — igniter resistance, flame sensor current, gas flow verification — is professional work on the ignition train, and it is quick work for a technician because the five-strike consistency means the fault will show itself on the first commanded cycle.
The same fault on other brands
Running out of ignition retries is the single most standardized event in furnace diagnostics — nine brand families on this site code it. Carrier's code 14 calls it ignition lockout, Goodman's 1 flash is lockout after failed attempts, Trane's 2 flashes is retry exceeded, and Rheem's *RC boards blink 4 after five failed tries — the same count Lennox uses. Lennox's contribution is the vocabulary: Watchguard, a soft lockout that retries on its own rather than staying down.
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.