On my Combi boiler the pilot light automatically lights when the hot water tap is on or the heating is switched on. But its stopped, then boiler tries to light (i can hear it) but then it cuts out and I have to reset (by pushing a small red button on the front of the boiler).

Mark, December 2006

You need to be a bit more specific with the model of Vaillant that you have, but from what you are saying, see if you can see the spark, if you can't - Then you may need new HT Leads and Electrodes... If you can clearly see a spark, make sure the gas is on... Then it could possibly be a PCB or Gas Valve but it's really impossible to say without knowing exactly what boiler it is and without being there.

GasAid.co.uk, December 2006