Field pattern: furnace lights, runs briefly and cycles off (filter and flame sensor before the board)
Two cheap causes account for most of these calls: restricted airflow that trips the high limit, and a coated or misplaced flame sensor that stops proving flame. The common wrong turn is reading 'ignition failure' or 'limit' on the LED, blaming the board and replacing it, while the filter, the sensor rod or the burner ground was the real problem.
Cross-checked ~45 min moderateBrands: All
Before you start
- Turn off power at the furnace switch before removing the burner or blower door; the door switch is not a lockout
- Do not jumper or bypass the limit, rollout or pressure switches to keep the furnace running
- Gas valve, manifold pressure, burners and heat exchanger work are BLOCKED: refer to a licensed gas technician
- If you smell gas, stop, ventilate, leave and call the gas utility
Diagnostic steps
Unlock all 8 steps with expected values
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