Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the connector plug marked X1 from the PCB
Measurement:
Test 1: Measure the resistance across the Brown wire (potentiometer wiper terminal, middle terminal) and the Blue wire at the CH Potentiometer(Boiler Thermostat). With the potentiometer turned fully down (anti-clockwise) the resistance should be short circuit (or very near short circuit).
Increase the potentiometer (clockwise) and observe the resistance increasing to a maximum of approximately 10 Kohms.
Test 2: Measure the resistance across the Brown wire (potentiometer wiper terminal, middle terminal) and the Black wire. The resistance should now be approximately short circuit (stat at Maximum).
Turn down the potentiometer fully (anti-clockwise) and the resistance should increase to approximately 10 K ohms.
Increase the potentiometer (clockwise) and observe the resistance increasing to a maximum of approximately 10 Kohms.
Test 2: Measure the resistance across the Brown wire (potentiometer wiper terminal, middle terminal) and the Black wire. The resistance should now be approximately short circuit (stat at Maximum).
Turn down the potentiometer fully (anti-clockwise) and the resistance should increase to approximately 10 K ohms.
Isolate as per TOPs P4
and refit as necessary