Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect plug from the flue thermistor
(If the appliance is not twin flued access can be improved by removing the three screws holding the panel at the top of the boiler)
Disconnect connector X401 from the PCB
(If the appliance is not twin flued access can be improved by removing the three screws holding the panel at the top of the boiler)
Disconnect connector X401 from the PCB
Measurement:
Check the connectors at the air pressure switch are connected securely.
Check for continuity (zero resistance) between
each of the two wires to the flue thermistor
(Pins 9 and 10 on X401).
Inspect the full length of harness and the
connectors for any signs of damage.
Check for continuity (zero resistance) between
each of the two wires to the flue thermistor
(Pins 9 and 10 on X401).
Inspect the full length of harness and the
connectors for any signs of damage.
Isolate as per TOPs P4
and refit as necessary