is it a valve thats in reasonable condition? if so take off the head and you will see a small pin that i think is stuck and not moving when you select a number on the valve head. i have removed the pin in the past on a couple of valves and a small/tiny amount of water seeped out. i cleaned the pin which had crap on it and re inserted it into the valve, and that did the trick.cheaper than a replacement valve.
if you do have to purchase one you are looking at draining down so its worth trying this first