Vokera compact he firing fault?

Hi, iv go a vokera he fitted. Its never been great, but recently it wont fire up. Iv had the cover off, checked all the connections and cleaned everything up! Now....it fires up...with the airbox cover off. Put the aibox cover back on and the burner cuts out. Am i right in thinking its the differential pressure switch that needs replacing?! Any other suggestings?

Thanks in advance,

Rich