volkera linea plus '08' fault code?

we have a volkera linea plus boiler that keeps getting '08' fault code

we did original get an '01' code so the enginner fitted a new air pressure switch but now we keep getting an '08' code.

enginner has stripped the burner and flue and still cant get tot he bottom of the problem. we keep resetting it and it only stays on for about 20 minutes and our tenants are getting pretty annoyed!

any help/advice/ideas much appreciated