Set Appliance State:

WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Service mode procedure:
Rotate the CH temperature selector to the spot on the facia, then turn it to full and back to the spot quickly.
The display shows 'SERV' and the appliance water temperature.
Note.
To take out of service mode, repeat the above process.
Turning the mains off then on again will also take it out of service mode.