Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect CN9 from the PCB

Measurement:

Test 1:
With no hot water tap open, measure the resistance across connections 7(black) & 8(black) of connector CN9.
Expected resistance will be open circuit.
Test 2:
Open a hot water tap (minimum flow rate 2 litres/min), measure the resistance across connections 7(black) & 8(black) of connector CN9.
Expected resistance should be short circuit.
Note.
Ensure a good connection is made.