Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4

Observation:

Check Air Pressure Differential Switch Tubes for any signs of physical damage.
NOTE: Tubes must be connected correctly:-
Top Venturi (-ve) connected to the front tapping of the Air Pressure Switch.
Bottom Venturi (+ve) connected to the rear tapping of the Air Pressure Switch.