Set Appliance State:
Set Multimeter to Resistance 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Lower front panel of control box to gain access to PCBs
Unplug connector X1 from main driver board
Lower front panel of control box to gain access to PCBs
Unplug connector X1 from main driver board
Measurement:
Test 1:
With hot water tap fully open measure resistance across brown and blue wires at unplugged connector.
Resistance should be short circuit.
Turn off tap.
Test 2:
With hot water tap turned off repeat resistance check across brown and blue wires.
Resistance should be open circuit.
With hot water tap fully open measure resistance across brown and blue wires at unplugged connector.
Resistance should be short circuit.
Turn off tap.
Test 2:
With hot water tap turned off repeat resistance check across brown and blue wires.
Resistance should be open circuit.
Isolate as per TOPs P4
and refit as necessary