Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Ensure that the boiler is not at its full operating temperature
Ensure the CH overheat thermostat trip switch is set
Ensure that the boiler is not at its full operating temperature
Ensure the CH overheat thermostat trip switch is set
Measurement:
Measure the resistance across the two red wires at terminals
OH and OH, on the terminal block to the rear of PCB 26.
OH and OH, on the terminal block to the rear of PCB 26.
