Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Isolate the cold water supply to the appliance
Drain the hot water circuit
Remove the flow meter assembly

Observation:

Inspect the flow meter magnet within the flow meter assembly,
it should move up and down freely and be in good condition.
Note.
You can test the magnet by using a screwdriver to see if
the magnet energises.