Why Your Furnace Lights Then Shuts Off in Seconds
One of the most common furnace complaints in Minnesota: the furnace lights, runs for a few seconds, then shuts off—over and over. This cycle is almost always caused by a dirty or failing flame sensor. The flame sensor is a metal rod positioned in the burner flame that passes a small electrical current through the flame to ground. When present, ionized combustion gases allow current to flow, confirming to the control board that the burners are lit. Without this signal, the board shuts off the gas valve within seconds as a safety measure.
Symptoms of a Dirty or Failed Flame Sensor
- Furnace lights briefly (2-7 seconds) then shuts off
- Furnace attempts ignition 3 times then locks out with an error code
- Error code indicating "flame sense fault" or similar
- Furnace works after a reset but shuts down again after a few cycles
DIY Flame Sensor Cleaning
Cleaning a flame sensor takes about 15 minutes and costs nothing but a piece of fine steel wool (#0000) or fine-grit emery cloth. Turn off power at the disconnect switch and shut off gas. Locate and remove the sensor (typically one screw, plus disconnect the wire connector). Gently polish the metal rod to remove white oxidation—a few light passes is enough. Reinstall, restore power and gas, and test through a full heat cycle. This cleaning often resolves intermittent issues for a season or two. If the problem returns within weeks, replacement is needed.
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Flame Sensor Replacement
Replacement sensors cost $8-25 and follow the same steps as cleaning, just swapping in a new unit. If you're not certain the flame sensor is the problem, a professional diagnostic is worthwhile—other issues like pressure switch faults or ignitor failures produce similar symptoms.
Professional Service Cost in Minnesota
If you hire an HVAC technician: service call/diagnostic $80-120, cleaning often included in service call fee, replacement sensor plus labor $100-200 total. During peak winter season, emergency service runs significantly higher. Annual flame sensor cleaning is typically included in professional tune-ups.
When Repeated Flame Sensor Issues Signal a Bigger Problem
A sensor needing frequent cleaning every few months may indicate incomplete combustion or a burner problem. Soot buildup (black deposits) versus normal white oxidation suggests burners aren't burning cleanly—which can also stress the heat exchanger over time. Have a technician inspect the entire burner assembly if this pattern repeats.
When to Consider Furnace Replacement
A flame sensor issue alone is not a reason to replace your furnace. But if your furnace is over 15 years old and experiencing multiple component failures, explore Furnace Direct's factory-direct Goodman furnaces—a new 96% AFUE two-stage unit starts around $850 delivered throughout Minnesota, with total installed cost typically under $2,500. Learn more about furnace lifespan by brand to evaluate your situation.
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