Furnace shows F10, no heating, pump may run briefly then stop

Safety first: Isolate power and allow the system to cool. Some water spillage may occur when working on the hydraulic block.

• Bleed system: Remove air from radiators and the pump. Low flow due to airlocks can prevent the flow sensor from operating.
• Check pump operation: Ensure the circulation pump runs when there is a heat demand. If the pump is not running, address pump issues first.
• Inspect flow sensor: Locate the flow sensor or flow switch on the hydraulic block. On some Saunier Duval models, it is a turbine or paddle-type sensor. Remove and inspect for debris or limescale. Clean gently and reinstall with new seals if necessary.
• Test microswitch: If the sensor uses a mechanical microswitch, test continuity while manually actuating the switch. Replace the switch if it does not change state reliably.
• Wiring check: Verify continuity of the harness from the flow sensor to the PCB. Repair or replace damaged wiring.