Furnace displays F1 and will not start, fan may run briefly then stop, no ignition sequence

Safety first: Isolate electrical power and close the gas valve. Allow the appliance to cool fully before touching any hydraulic components.

• Locate the flow NTC: On Saunier Duval furnaces, the flow temperature sensor (NTC) is usually clipped or screwed into the flow pipe leaving the primary heat exchanger. Consult the service manual for exact location.
• Check wiring and connector: Gently unplug the NTC connector from the harness, inspect for corrosion, bent pins, or moisture. Clean with electrical contact cleaner and reconnect firmly.
• Measure resistance: Using a multimeter, measure the NTC resistance at room temperature and compare with Saunier Duval specifications (typically in the 10–20 kΩ range at 25°C, but verify in the manual). An open circuit or near-zero resistance indicates a failed sensor.
• Inspect for overheating: Look for signs of overheating or limescale on the flow pipe around the sensor. Heavy scale can cause localized overheating and repeated F1 faults; the system may need flushing by a professional.
• Replace if faulty: If the NTC is out of spec, replace it with a genuine Saunier Duval flow NTC sensor. Refill and bleed the system if any water was lost, then reset the furnace and test operation.