Sprinkler trouble - where to dig?

The heads on one of the zones on our sprinkler system flow very slowly, the heads don't rise - just a little water seeps out. All the heads in the zone are the same and I've checked the controller so I figure it's time to go digging and find the control valve. BTW the system is controlled with small plastic water lines not electrically. My question is how do you decide where to start digging?
TIA
Peter
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Begin with the easiest, the obvious. Take the heads off and check the screen if it has one. See if you can blow through the nozzles. With the heads off, turn the valve on and off to flush any dirt/rocks out of the line. Put the heads back on except for the last one. Turn it on and off again to blow the last crud out the last hole. See if that will fix the problem.
If you can't find blockage, or blow it out, then is the time to start digging. Digging is the last thing I would do.
Steve
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