Furnace lights, then dies seconds later — the dirty flame sensor (any brand)
Applies to: Gas furnaces, all brands (Carrier, Trane, Lennox, Goodman, Rheem and others)
What this error means
The single most common furnace failure across every brand, and it costs nothing to fix. The flame sensor is a thin metal rod that proves the burners lit; when coated with combustion residue it can't sense the flame, so the board — believing ignition failed — shuts the gas off after a few seconds. Three to five failed tries and the furnace locks out (Carrier Code 34/14, Trane 2 flashes, Lennox Watchguard/E270 — all roads lead here).
Symptoms
- Burners ignite, run 3–10 seconds, shut off
- Pattern repeats several times, then lockout
- Happens more at the start of heating season
Try this first (safe DIY steps)
- Kill power at the furnace switch and close the gas valve
- Find the flame sensor: a thin single-rod sensor held by one screw, positioned in the path of the last burner (it is NOT the igniter — the igniter is usually flat/forked)
- Remove it, polish the rod gently with fine emery cloth or a dollar bill's worth of light abrasion until bare metal, reinstall
- Restore power; the furnace should hold flame. Total time ~20 minutes
- If it fouls again within months, just replace the sensor — they're $10–$30 parts
Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.
When to call a technician
If a clean sensor doesn't fix it: igniter, gas pressure, or board diagnosis — standard service call $80–$250.