an error is usually a bad solenoid, which creates a short. a broken wire is an open, which usually does not result in an error at the clock.
as far as tracking, do everything you can to isolate the wire you are tracking. pull out the wire and any commons from the clock. unplug the timer, and disconnect any additional ground that is in place. on a residential job, one can usually narrow down possible vavle locations to smaller areas and concentrate there. then just start listening. also a lower power output from the locator will help with the signal bleed. good luck