Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Fit a wire link between the two right hand terminals at the mains inlet terminal block
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Does appliance now respond to a CH demand?
Note:
If appliance still does not respond to a CH demand leave link in place until diagnosis is complete.