Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
DHW temperature selector to high setting
Boiler On/Off switch Off
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler On/Off switch On.
Reset lockout if necessary.
Measure voltage across the flame sense electrode lead at the flame sense electrode and a suitable earth point.
Expected voltage, approximately 55 volts AC.
Note:
It can be difficult to test inside the rubber cover on the electrode, take care to ensure good contact is made.