Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect connector PL5 from the PCB
Set the boiler control thermostat located on the front panel
to the minimum temperature setting - ensure that the control
is NOT switched to the Off position

Measurement:

Measure the resistance between the orange and purple wire
terminals on the removed PL5 connector block Expected resistance approximately 95 KOhms.
Set the boiler control thermostat to the maximum temperature setting.
Repeat test. Expected resistance approximately 1 Ohm.
Note:
Test at disconnected connector block - do NOT test at the PCB.