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.

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.