Set Appliance State:
Gas Off
Set Multimeter to Resistance 
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Disconnect the Main PCB Connector PL8 from the PCB
Disconnect the Main PCB Connector PL8 from the PCB
Measurement:
Measure the resistance across the Red wire (connection 4 marked R out) and the Black wire (connection 5 marked R in) on the Main Terminal Block.
Note:
If no Room Thermostat is fitted there should be a link wire (1) connected between connections R out and R in on the Main Terminal Block.
Note:
If no Room Thermostat is fitted there should be a link wire (1) connected between connections R out and R in on the Main Terminal Block.
Isolate as per TOPs P4
and refit as necessary