Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the connector from the return thermostat which is situated to the right
of the combustion chamber, on the flow pipe to the primary heat exchanger.
of the combustion chamber, on the flow pipe to the primary heat exchanger.
Measurement:
Measure the resistance between the thermostat on the heat exchanger
flow pipe and a suitable ground point on the appliance (i.e. the return pipe).
flow pipe and a suitable ground point on the appliance (i.e. the return pipe).
Isolate as per TOPs P4
and refit as necessary