Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Locate the air pressure switch tubes
There are two air pressure switch tubes: one blue and one white
These run from the air pressure switch (situated at the back of the appliance in the top right-hand corner) and the fan (situated above the combustion chamber)

Observation:

Check the condition of the air pressure switch tubes for blockages, splits or poor condition.