I'll leave you to keep guessing as to what kind of engineer I am though, and in future I'll make sure to keep an eye on the forum rules too - just like everyone else!
My earlier post may have lacked my normal clarity.
The Forum Rules clearly state that we cannot give advice on gas/combustion related matters and thats why no on has given you the replies you hoped for.
The reality is that the supply voltages to fan etc are irrelevant as its the speed control signal which is more relevant. But those aspects are only checked when there is a fault with that part of the process. The first measurement is the combustion measurements and if incorrect then we work backwards from there.
The gas/air mix depends on the pressures and settings of the gas control components. On a premix boiler these are quite critical. Gas will only burn at all in a 5-15% gas/air mixture.
Tony