Lennox furnace Alert code 273: Flame failed in run mode — Watchguard
Alert 273 means the flame went out during a heating cycle and the system entered Watchguard mode. The furnace can light — it cannot keep the flame.
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
This is a distinctly different fault from a furnace that will not ignite. The igniter worked, the gas valve opened, and a flame was established. Something then either extinguished it or stopped the control detecting it. Lennox's checklist points firmly at flame detection: check microamps of the flame sensor using the thermostat or control diagnostics, clean or replace the sensor, measure voltage of neutral to ground to ensure a good unit ground, and clean the face of the burner assembly. The grounding check appearing again is notable — it recurs across Lennox's flame-related alerts and across other manufacturers' guidance too.
Most common causes
- A flame sensor coated with oxide, giving a signal that fades within seconds of lighting.
- Poor unit grounding, which Lennox has you verify by measuring neutral to ground voltage.
- A dirty burner assembly face.
- The flame sensor out of position or damaged.
- Low or unstable gas pressure, so the flame lights and cannot be sustained.
- Restricted combustion air destabilising the flame once established.
How to diagnose it
- Watch a start attempt if the furnace has a sight glass. Burners lighting and going out after a few seconds is the signature.
- Note how long the flame survives — a few seconds points strongly at flame sensing.
- Note when the furnace was last serviced, since an uncleaned flame sensor is the usual explanation.
- Note whether the furnace has a history of intermittent electrical faults, since grounding causes several at once.
How to fix it
- Flame sensor cleaning, grounding measurement, and burner cleaning are technician work.
- Tell the technician that the burners do light and then go out. It is the single most useful observation for this fault and it rules out the igniter immediately.
- Ask for the neutral to ground measurement Lennox specifies if a cleaned sensor has not resolved it.
- Do not reset repeatedly. Each cycle delivers gas that briefly burns and then stops.
Notes
A furnace that lights, burns for a few seconds, then shuts down is the classic dirty flame sensor across every manufacturer. It is among the most common furnace faults and one of the least expensive to repair.