Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB (use Insulation Mat as required)
Disconnect the link on the external controls terminal block
CH temperature control knob to MAX
Integral timer set to OFF
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?