Isolate Mains Power to appliance as per TOPs P4
Ensure appliance is switched off.

Observation:

Check that the Air Pressure Switch Tubes and adaptor connections to flue ducting are free and clear from blockages.
Additionally check that the Red Tube is connected to the +ve and the White Tube is connected to the -ve tappings on the Air Pressure Switch.
NOTE:
Early models may only have one air pressure switch tube this should be connected to the positive tapping on the pressure switch.