My friend has a Vokera Compact HE combi boiler.

When HW or CH is called for the boiler starts to make its normal firing up
noises (I assume it is the fan) but never actually fires. There is no
ignition sequence or spark.

I took the cover plate off the burner front to have a look at the spark and
with this cover off the boiler fires. Replace the cover and the boiler stops
burning. The fan stays running.

The flue is clear of any debris and I changed the air pressure switch and
checked the tubes for damage. No change.

I realise that the boiler fires with this cover off as air is getting into
the burning chamber through the front and not through the flue.

So where do I look next to find the fault?

Adam