Ferroli combi boiler

the problem with the boiler is when the room seal cover is fitted the dhw will not light but then goes immediately off and the ch will not light at all. I have renewed the airpressure switch which has made no difference. Please can someone shed some light on this problem

lack of air suggests flue/fan venturi check flue is assembled correctly or not blocked.what model is the combi?

I dont get the problem??

When the cover is off the DHW will light???

When the cover is on what does it do??

What made you change the pressure switch??

Help me help you!!

when the cover is on the ch will not try to light the fan comes on and thats all it does, as soon as the cover is removed the boiler lights, the boiler is a fer falcon (G.C No: 47-26701 W.R.C Ref: 98007) the problem is the same for the dhw but that will light for about 5 sec the go out, if i then remove the cover it will work straight away. The reason i change the air pressure switch was that our local corgi man said that was what the problem will be, but he has no more suggestion. The flue seems to be ok their is no obstruction within the pipework.