Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
Gain access to PCB, use insulation mat as required
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Locate connector M4 on the PCB and measure the voltage between pins B and C of the plug.