All in your manual or you can download one from internet,Manufacturers apparently think that you should know how your appliance works and how to fault find to a degree,that`s why they print a big fat manual,no qualifications needed. Gas safety is one thing,refusing to ask simple questions on faults on your gas appliance is pathetic. By the way, depending on boiler,most faulty components are outside of the combustion chamber so there is no reason why anyone cannot work on their own boiler or any boiler you choose for that matter. 
By the way, depending on boiler,most faulty components are outside of the combustion chamber so there is no reason why anyone cannot work on their own boiler or any boiler you choose for that matter.
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