Set Appliance State:
Use Insulation Mat as required
Set Multimeter to AC Volts 
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Selector switch to Heating & Water
Gain access to PCB
Selector switch to Heating & Water
Gain access to PCB
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure voltage between connection 4 (brown wire) and connection 2 (blue wire) on connector X19 on sequence board.
Isolate as per TOPs P4
and refit as necessary