Set Appliance State:

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

Measurement:

Is there Mains Voltage present between connector M3 connection 14 L (White) and Neutral N (Blue)?