Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Turn Hot Water Tap On
Summer/Winter Switch set to Summer.
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across pins 19 (brown) and 23 (white).
Note:
There is 110 V AC present when tap is off.