Set Appliance State:
Set Multimeter to AC Volts 
CH 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 output terminal (yellow wire that runs from the thermostat to the PCB) 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