Gas Furnace (standing pilot or hot-surface / spark ignition with flame rectification)
Flame Sensor Troubleshooting
Step-by-step diagnostic for gas furnace flame-sensing failures — from dirty sensors to grounding and control board faults.
Safety Notice
Disconnect power before handling wiring. Gas furnaces produce lethal voltages, combustion gases, and hot surfaces. Only trained, qualified technicians should perform these procedures. Follow NFPA 54 and local codes.
Flame Sensor Troubleshooting Overview
This workflow helps diagnose why a gas furnace ignites but loses flame shortly after. The most common cause is a dirty or failed flame sensor, but grounding, polarity, burner, and control board issues can produce the same symptom. Before starting: • Confirm the thermostat is calling for heat. • Confirm the inducer runs and the pressure switch closes. • Confirm the igniter fires and the burner lights. If the burner never lights at all, use the IFC or Pressure Switch workflows instead.
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