Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Unplug connector X1 from main driver board
Unplug connector X1 from main driver board
Measurement:
Test 1:
Measure resistance between brown and blue wires at the disconnected plug, resistance should be open circuit.
Test 2:
Leave probes connected and remove the 2 screws holding the proximity switch to the flow switch, slide the proximity switch down the flow switch.
The plastic switch must remain in contact with the flat brass face of the valve.
Measure resistance, resistance should be short circuit.
Note:
The proximity switch works on a magnetic contact between the flow switch and the proximity switch.
Measure resistance between brown and blue wires at the disconnected plug, resistance should be open circuit.
Test 2:
Leave probes connected and remove the 2 screws holding the proximity switch to the flow switch, slide the proximity switch down the flow switch.
The plastic switch must remain in contact with the flat brass face of the valve.
Measure resistance, resistance should be short circuit.
Note:
The proximity switch works on a magnetic contact between the flow switch and the proximity switch.
Isolate as per TOPs P4
and refit as necessary