Burners light, then shut off after a few seconds (flame sensor)

The board must prove flame by flame rectification through the flame sensor. A dirty or oxidized sensor, poor ground or reversed polarity makes the board shut the gas off right after ignition.

Single source ~30 min easyBrands: All

Before you start

If you smell gas: do not operate switches or the furnace, leave the building and call the gas utility from outside.
Make sure the home has a working carbon monoxide (CO) alarm; if it is sounding or anyone has headache/nausea, get everyone out and call 911 / the gas utility.
Turn off power at the furnace service switch or breaker before opening the burner or blower compartment; the door switch alone is not a lockout.
Gas-side diagnosis or repair (gas valve, gas pressure, burners, heat exchanger, venting) is for a licensed gas/HVAC technician only.
Never jumper or bypass a limit, rollout or pressure switch.

Diagnostic steps

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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