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A dirty flame sensor is a common reason a furnace lights for a few seconds and shuts off. We clean and test the sensor, then confirm the furnace is proving flame safely.
The flame sensor is a safety part. It tells the control board that burners actually lit. If the board does not see a clean flame signal, it shuts the gas valve.
Cleaning helps when oxide buildup insulates the sensor. Replacement is sometimes needed, but we do not replace it before checking grounding, wiring, burner flame, and the control board reading.
A real diagnosis includes microamp flame signal testing. If somebody cleans the rod and leaves without testing the signal, they have not proved the fix.
$89-$150 typical
The sensor gets coated over time and stops carrying a reliable flame signal.
Diagnosis required
A dirty burner or gas pressure issue can make the sensor look guilty when the flame is the real problem.
$120-$250 typical
The flame signal uses the furnace ground path. Loose grounds cause intermittent lockouts.
$350-$650 typical
Less common, but possible after the sensor and wiring test correctly.
Do not remove burners or bypass safety wiring. Basic checks are fine; safety controls are not DIY parts.
Replace the filter if it is packed with dust.
Write down any blink code before resetting power.
Confirm the gas valve handle is parallel with the pipe.
Stop if you smell gas or hear delayed ignition bangs.
Many Northwest Side basements double as storage, laundry, and mechanical space. Dust, lint, and renovation debris all end up near the burner compartment.
That does not mean every flame sensor call is just dirt. We still check the whole ignition sequence because a dirty sensor and a weak ignitor can show up together on older equipment.
If this page is close but not exactly your problem, these pages may match what you are seeing.
Some homeowners do, but it requires shutting power off, removing the sensor without damaging it, and reinstalling it correctly. The missing step is testing flame signal afterward.
Many furnaces go a few years between cleanings. If it needs cleaning every season, something else may be wrong with combustion, grounding, or burner alignment.
No. A dirty sensor is a normal service issue, not a replacement reason by itself.
Bernie's cleans, tests, and verifies the ignition sequence before selling parts you may not need.