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.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
As lockouts go this one is a bargain, because its most common cause has the cheapest fix in the repair guide: flame sensor cleaning at $80–200, the ten-minute job the guide calls the classic easy fix. The escalation from alert 240 matters for the wallet mostly as urgency — 240 is the same weak signal reported while the furnace still limps along; 273 means it has now cost you heat, repeatedly, so the visit stops being optional. Budget realism: if the sensor is cleaned and the lockouts return within weeks, the next tier is electrical — the neutral-to-ground check Lennox specifies, hunting the poor ground that makes clean sensors read dirty — and that is diagnostic time at $75–150, not another cleaning.
DIY or call a pro
The flame sensing circuit is where this site draws its hard DIY line, and alert 273 is entirely about that circuit, so the hands-on portion belongs to a technician — including the sensor cleaning itself, which happens at the burner assembly. The homeowner leverage is pattern evidence, and for this alert it is specific: how many minutes into a run the flame drops (consistent timing suggests a developing condition; random timing suggests electrical), whether it is worse on windy days (vent recirculation degrades the flame), and how many times this has happened before. That last one is the misdiagnosis alarm: a furnace on its second or third sensor cleaning with lockouts returning each time is showing the signature of a grounding fault, not a dirty sensor — say exactly that when booking, and point the technician at alert 117 if it appears anywhere in the fault history.
What this looks like
From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — that walkthrough covers what to check in order, and when it stops being a homeowner job.