Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to PCB (use Insulation Mat as required)
Disconnect the wires running to the external controls and make safe
Ensure No demand for CH or DHW
CH temperature control knob to MAX
DHW temperature control knob to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Does the pump still run / boiler still fire with no demand?