Furnace shows F2, burner shuts down shortly after starting or does not start, may cycle on/off rapidly

Safety first: Disconnect power and close the gas valve before removing covers. Allow the hydraulic circuit to cool.

• Identify the return NTC: The return temperature sensor is mounted on the return pipe entering the primary heat exchanger or hydraulic block. Refer to the Saunier Duval schematic for your model.
• Check connections: Inspect the NTC plug and cable for damage, kinks, or signs of overheating. Reseat the connector and ensure it is firmly latched.
• Test the sensor: With the sensor disconnected, measure its resistance at ambient temperature and compare to the manufacturer’s resistance/temperature table. If the reading is far outside tolerance, the sensor is faulty.
• Check for swapped sensors: On some models, flow and return NTCs are identical. If they were previously replaced, verify that each is connected to the correct harness plug as per the wiring diagram.
• Replace if needed: Install a new Saunier Duval return NTC sensor if defective. Clear the fault by resetting the appliance and verify stable operation under heating demand.