Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
Selector switch in CH position
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage on connector J1 connection marked 1 in diagram (this is the input side of the external controls) and connection marked N (blue wire).
Note:
If no external controls are fitted then a link should be present between connections marked 1 & 2 in diagram.