I have been servicing a vokera maxin 28e boiler,all the work is complete and all the pressures are correct to manufacturers instructions,the boiler will go through all the correct sequence and fire up when the burner chamber cover is removed but the CO reading is high but when i put the burner chamber cover back on and open the hot water tap or switch on the heating the boiler will go through all the start up process except for the firing part it will not ignite, what could be the problem please as there is no clue in the installation manual

jay, October 2010

From description cover off, ignites, cover on doesn't, would lead me straight to the fan pressure switch.

However, if you are getting high CO readings it could be corruption in the flue seals (also stopping the fps from operating), check in the air intake test point with the boiler running (rather than the flue outlet test point), you should be getting O2 readings of approximately 20.9% on air intake.
Any less than this suggests the flue seals are corrupt and 'mixing' could be occuring in the flue, this would obviously warrant further investigation into the flue.

Hope this leads you in the right direction.

Carefree, October 2010