Set Appliance State:
Mode selector switch to Off/Reset position
Set Multimeter to AC Volts 
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Hot water temperature control set to maximum
Gain access to PCB (use insulation mat as necessary)
Hot water temperature control set to maximum
Gain access to PCB (use insulation mat as necessary)
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
With the connector still fitted to the PCB, allow the boiler to fire.
Continually measure the voltage across live connection (brown wire) on connector M14 on the main PCB and the neutral (blue wire) on connector M14 on the main PCB until the boiler cuts out.
Continually measure the voltage across live connection (brown wire) on connector M14 on the main PCB and the neutral (blue wire) on connector M14 on the main PCB until the boiler cuts out.
Isolate as per TOPs P4
and refit as necessary