usually its just the head that burns out so a replacement head is merely 2 screws and a few wires to connect in as old one was wired. Simples!
If the valve itself has siezed/snapped off/ etc then its a drain down to replace full assembly.
Caution as some older valves you cant remove head without a flood =) as they not designed to be split, if in doubt the easiest way for you, would be to phone a parts center and ask if a replacement head is available for the (make and model) valve head you have.
ste
October 2009