Set Appliance State:
Set Boiler Control Thermostat to MAX position
Set Multimeter to AC Volts 
DHW Demand
Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB
Use insulation mat where necessary
External controls calling for heat
Gain access to PCB
Use insulation mat where necessary
External controls calling for heat
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the voltage across the boiler control thermostat input terminal (yellow wire that runs from the overheat thermostat to the boiler control thermostat) and the neutral (marked N) on the main connector block.
Expected voltage 230 volts AC (mains voltage).
Expected voltage 230 volts AC (mains voltage).
Isolate as per TOPs P4
and refit as necessary