I suffered this with a Potterton Boiler recently and have discovered that Boiler Engineers usually say - Defective Gas Valve and this may well be the case. I assume their experience tells them it usually is the Gas Valve but it ould be other things...
Pilot injector partially blocked resulting in a weak pilot flame generating insufficient heat to trigger the next step in the control electronics.
Defective PCB failing to open the Gas Valve.
When your boiler engineer arrives the first thing he should check is if the correct voltages are being presented to the Gas Valve solenoid to open it - if they are correct the Gas Valve really is defective - if the correct voltage is not present then the GV is probably good.
Peccavi, April 2009
