Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X1 from the PCB
Boiler thermostat set to MAX

Measurement:

Test 1:
Measure the resistance between pin 1 and pin 2 at connector X1.
Expect continuity / zero resistance.
Test 2:
Measure the resistance between pin 2 and pin 3 at connector X1.
Expect approximately 0.9MOhms.
Turn the control knob to MIN.
(as low as the thermostat will go without clicking into the off position)
Test 3:
Measure the resistance between pin 1 and pin 2 at connector X1.
Expect approximately 0.9MOhms.
Test 4:
Measure the resistance between pin 2 and pin 3 at connector X1.
Expect continuity / zero resistance.
Are the resistances measured as expected?