Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect the black wires that run from the flue spillage sensor at the inline spade connectors (the wiring colour changes from black to green at the connectors)
Disconnect the black wires that run from the flue spillage sensor at the inline spade connectors (the wiring colour changes from black to green at the connectors)
Measurement:
Measure resistance across the black wires that run to the spillage sensor resistance should be between 1 - 30 kOhms.
Note:
The spillage sensor is a safety feature of the appliance if flue spillage is detected the appliance is shut down and a 20 minute blocking time is initiated.
Note:
The spillage sensor is a safety feature of the appliance if flue spillage is detected the appliance is shut down and a 20 minute blocking time is initiated.
Isolate as per TOPs P4
and refit as necessary