Boiler displays F0 and does not start heating or producing hot water

Safety first: Turn off the boiler at the main switch and isolate the gas supply. Allow the boiler to cool before opening any panels.

Steps to check:

• Remove the front casing according to the Saunier Duval manual to access the hydraulic block and flow sensor.
• Locate the domestic hot water (DHW) flow sensor/flow switch on the cold water inlet side of the boiler.
• Inspect for visible scale, debris, or damage. Clean the filter or strainer if fitted.
• Check the electrical connector: ensure it is fully seated and that the wires are not damaged or corroded.
• With power off, test the sensor with a multimeter (refer to the service manual for correct resistance or switching values). Replace if readings are out of specification.
• Reassemble, restore power and gas, then open a hot tap to see if the code clears and the boiler fires.

Warning: Do not work on live electrical parts. If you are not confident using a multimeter or removing the casing, contact a qualified heating engineer.