Set Appliance State:
Gas Off
Set Multimeter to AC Volts 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Selector switch in HW position.
Gain access to PCB as necessary
Selector switch in HW position.
Gain access to PCB as necessary
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure voltage across Domestic Hot water Thermostat (Black wire) and Neutral, (Blue) on Appliance Terminal Block.
Isolate as per TOPs P4
and refit as necessary