Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
DHW Taps OFF
Measurement:
Disconnect the flying lead of the flow switch (Black wires) from the Blue and Brown leads which run to the PCB. Measure the resistance across the disconnected Black wires of the Flow Switch.
Reconnect the leads after the test.
Reconnect the leads after the test.
Isolate as per TOPs P4
and refit as necessary