Well he may be correct - the gas valve is really two separte valves I think - one valve for the pilot flame and another for the main burner - both built into the same metal block.
When the boiler is called on to heat water the fan runs and a pressure switch signals the control board that it's working - then the pilot flame is lit and when the heat sensor tells the PCB that the pilot is good the PCB opens the main gas valve. You can see the pilot flame through the sight glass.
It may be exactly as the service man says but I have a suspicious mind - what a coincidence it failed immediately after a visit and why did he not demonstrate the "strange noise" to your mother and quite how a defective PCB or a Gas Valve can cause a boiler to make strange noises I can't imagine.
I would call in a different engineer - it might just be a slow running fan or a pressure switch or even a bad case seal.
A new gas valve is in the order of £160 plus VAT and a PCB £120. Fan £135 Pressure switch £45 - such obscene prices - a scandal.
Good luck...
Peccavi
October 2009
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