Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Selector Switch in HW position.
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Is there Mains Voltage across connector M2 connection 5 (Blue wire) and connection 6 (Brown wire) on the main PCB?