Set Appliance State:

Set Multimeter to AC Volts Set meter to measure AC Volts
Isolate Mains Power to appliance as per TOPs P4
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Drop the facia and remove the 6 screws from the PCB housing
Turn over housing to reveal the PCB
Use electrical insulating mat
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure for AC volts on connections 2 and 3 on Plug X8,
best contact can be made on the terminals direct on the PCB.
Expect approx. 2 V AC with DHW demand.
Remove the demand and the voltage should fall to approx. 0 V AC.
Notes:
Place the probes in the small holes on the PCB.
Ensure a good contact is made otherwise misdiagnosis will occur.
If in doubt repeat the test.