Furnace locks out with 02, fan runs but burner does not light

Safety: Isolate electrical power and close the gas valve before opening the casing.

• Check for intermittent gas supply: Confirm other gas appliances run without interruption. If they also cut out, contact the gas supplier.
• Inspect flame sensor and earth: Remove the combustion cover. Locate the flame sensing electrode. Clean the metal tip with fine abrasive pad and ensure the mounting bracket has a clean metal-to-metal contact with the burner for proper grounding.
• Check wiring harness: Trace the flame sensor lead back to the control board. Look for damaged insulation, loose spade connectors, or signs of overheating. Reseat all connectors.
• Observe operation: Restore power and gas, then call for heat. Watch through the sight glass: if the flame lights and then drops out after a few seconds, the sensor or PCB may be faulty.
• If lockouts persist: Do not repeatedly reset. A qualified technician should test flame rectification current, gas valve stability, and PCB operation.