How to Fix / Troubleshooting

Safety first: Isolate electrical power and close the cold water inlet valve before removing any hydraulic components.

Steps:

Check for flow: Open a hot water tap fully and verify that water actually flows from the tap. If not, there may be a plumbing issue upstream.
Inspect flow sensor: Locate the Saunier Duval DHW flow sensor on the cold inlet. Check the electrical connector is firmly seated and wires are intact.
Clean or replace turbine: Shut off water, relieve pressure, and remove the flow sensor body. Inspect the turbine wheel for scale or debris. Clean gently and ensure it spins freely. Refit with the arrow in the correct flow direction.
Test operation: Restore water, bleed air, then power. Open a hot tap and see if the unit detects demand. If F6 persists, the sensor or PCB input may be defective.

Warning: Always check for leaks after refitting hydraulic parts. Do not overtighten plastic fittings.

Display shows F6, water heater does not start, often after sensor wiring work

Possible Causes