Furnace attempts to start but shuts down with E02, fan may run briefly, no stable flame.

Safety: Switch off electrical power and close the gas isolation valve before opening the furnace casing.

Steps to try:

• Check for drafts: Ensure there are no strong drafts or open panels around the furnace that could blow the flame off the burner, especially in utility rooms or garages.
• Inspect burner for blockage: Remove the burner cover and visually inspect the burner ports for rust, dust, or debris. Use a soft brush and vacuum to clean the burner surface. Do not enlarge burner holes.
• Clean and reseat flame sensor: Remove the flame sensor, clean it with fine emery cloth, and ensure the mounting screw is tight and the sensor is properly grounded to the burner frame.
• Check wiring harness: Inspect the wiring from the flame sensor back to the Cointra control board for loose connectors, damaged insulation, or corrosion. Reseat all plugs.
• Monitor gas supply: If other gas appliances flicker or go out, there may be a supply or regulator issue. Contact your gas provider.

Professional checks: A technician should verify gas inlet and manifold pressure, check the flame signal (µA reading from flame sensor), and test the main control board. If the flame signal is weak despite a good sensor and clean burner, the control board may need replacement.