Error F.28 on Vaillant?

There is no 'pilot light' it is an automatic ignition. F.28 indicates failure on ignition ie: no gas. First check you have gas and it is turned on (at the meter), once you have established this, there could be a variety of causes.
Gas valve.
Burner partial blockage (lights but fails to establish flame).
Lack of water pressure has also been known to cause this fault.

I can only advise you that (before you go 'fiddling' with gas appliances), you MUST be deemed to be competent to do so, especially when playing with gas components.

By your initial post asking about pilot lights, I can only assume you are not. Therefore, don't attempt repair yourself.

Carefree
July 2010