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Furnace lights then goes out after 3-10 seconds? It's usually the flame sensor. Diagnose and fix it step by step.
If your furnace lights for 3-10 seconds then shuts off, the flame sensor is the culprit 80% of the time. The flame sensor is a simple metal rod that detects flame by passing a tiny electrical current (1-5 microamps) through the flame to ground. When it gets dirty, the signal drops below the board's threshold and the board shuts off the gas valve as a safety precaution.
Turn off power. Remove the single screw holding the sensor. Gently sand the metal rod with fine emery cloth or steel wool until shiny. Reinstall. This 5-minute fix solves the problem 80% of the time. If it doesn't, replace the sensor — they're $10-$25. See our error code guides for brand-specific troubleshooting.
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