HVACopediaLennox · Furnace troubleshooting
Burners light, then the furnace shuts down early
What is going on
The burners light but the furnace shuts off before the thermostat is satisfied.
Likely causes
- Restricted exhaust or low pressure differential at the prove switch
- Wrong concentric vent kit
- Condensate line not draining
- Flame sensor installed wrong, broken or grounded, or not sensing flame
- Rollout tripping from overfire or heat exchanger problems
- Flue gas recirculation or damaged crossovers
Check in this order
- Turn off power at the disconnect before touching wiring or components.
- Check the LED pattern. Pressure switch, watchguard and rollout patterns each point to a different branch.
- Check the exhaust for blockage, vent size, the vent termination kit and the condensate line slope.
- Check the flame sensor location, sense wire continuity and shorts to ground. Clean the rod with steel wool, not sandpaper.
- If the rollout trips, check manifold pressure and orifices against the nameplate, then the heat exchanger and burner position.
- If ignition is rough and flames drop out, check the intake and exhaust for recirculation and inspect the burner crossovers.
Applies to: G43UF-24B-045 through G43UF-60D-135 (SureLight boards 32M88, 78M47, 100973-01)
More furnace guides
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- EL296UHV: normal gas heat sequence (what should happen after a call for heat)
- EL296UHV: what the display and the diagnostic button are telling you
- Furnace will not run (no heat) - Lennox EL195UHNE
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Source: Lennox G43UF Unit Information Corp. 0416-L4, Troubleshooting Chart Problem 5 pp.53-54. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.