Set Appliance State:

Mains Power Off
Mode selector switch to Off/Reset position
Set Multimeter to AC Volts Set meter to measure AC Volts
Central heating temperature control set to maximum
WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Mode selector switch to CH position.
Measure the voltage across the connection 3 (white wire) on connector M16 on the main PCB and a suitable neutral.
Note:
Connections are numbered top to bottom.