Boiler problem

Can anyone please help me???
I have a ferolli 77 cobi boiler and the pilot is on but when the heating or hot water is turned on the internal fan starts up but the burner doesnt ignite. Can anyone tell me a simple proceedure to establish what is wrong.
Many thanks in advance
Marc
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could be a number of different problems air pressure switch or thermister fault seems most likely

or
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The gas valve is pretty common to go too. Not cheap if it is.
Christian.
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