Diagnosis - a process that heating engineers sometimes use.
The only absolute way to diagnose a faulty Circuit Board is to measure the input signals and voltages and to compare them with the expected outputs for each condition.
Your fault may indeed be the PCB but there are other things to check first - the installation manual for boilers often has a fault finding chart in the back.
Could the sound be the noise of a fan running? You should hear a fan. A pressure switch tells the control board that the fan is running OK - the control board fires up the boiler.
Good luck...
Peccavi
July 2009
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