I am assuming that the system is run from a multi-station timer. I would look for a short in the area where the zone wires are connected to the timer. I do not have a clue how to do this except by wiring up a new timer and see if this solves the problem. Hope that my input is of some practical value.

Thanks for the input. I had a revelation this morning on the ride in. I have a master valve in front of the ganged valve manifold. I suspect my zone 6 valve is just stuck open. It only appears to shut off because the master valve shuts off the flow to the entire manifold. My self-installed system is 12 years old now - I suspect the diaphragms and springs in the valves are probably in pretty rough shape. I'll replace the inners of the bad one (and check the solenoid). If that corrects the problem I'll probably replace the inners on other five.

Bingo! That's what I was going to recommend! The key was that the zone only failed open when the system ran which would indicate that the system had a master valve. It's a hydraulic issue, not an electrical problem. Replace the guts and solenoid while you're at it if you can match them up with your 12 yr. old valve and you should be golden.

All Wet
Texas Licensed Irrigator #1897 since 1984

Thanks All Wet. I had a few spare valves, plus found a place on line that had 10 in inventory - bought them all :) My valves are no longer made, but I should be good for another 20 years now with the additional 10 purchased.