The burners roar to life, run for two or three seconds, and shut down. Again. And again. Before you brace for a big repair bill: this exact symptom has one overwhelmingly common cause, and it's a $12 metal rod.
Meet the flame sensor
Your furnace proves the burners actually lit by passing a tiny current through the flame to a sensor rod. When that rod gets coated in oxidation — which happens to every furnace, every few years — the signal drops, the control board assumes there's no flame, and it slams the gas valve shut. Light, die, retry, lockout.
The 10-minute fix
- Kill power and gas to the furnace
- Find the thin rod with one wire, mounted in the burner path (opposite the igniter)
- Remove the mounting screw, slide it out
- Polish gently with fine sandpaper or an emery cloth — never bare-hand the rod afterward
- Reinstall, restore power, test
If cleaning buys you weeks but not months, the rod's done: a universal flame sensor costs about what lunch does, and swapping it takes the same 10 minutes.
When it's NOT the flame sensor
If the furnace never ignites at all (no whoosh, no glow), you're in igniter or gas-valve territory. If it runs minutes before dying, look at limit switches and airflow. Our free flame sensor troubleshooter walks the whole decision tree with you, microamp readings and all.
🔥 Run the Flame Sensor Troubleshooter →
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