Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connectors from DHW flow sensor
Turn DHW tap On with a flow rate set @ 3 litres/min.

Measurement:

Measure for resistance across the flow switch terminals.
Expected resistance continuity (zero resistance).
Note.
Ensure a good connection is made or misdiagnosis will occur.