A sprinkler system that has normal pressure until a zone opens may have a leak, restriction, valve problem, excessive flow demand or supply-side issue. The pattern matters: one weak zone points toward a different set of causes than every zone being weak.
If only one zone loses pressure
Check for broken heads, underground lateral-line leaks, a valve that is not opening fully, clogged nozzles or too many high-flow heads on the zone. Our low-pressure troubleshooting process compares zone behavior rather than assuming the water supply is the problem.
If every zone loses pressure
System-wide weakness can point toward the irrigation shutoff, backflow/supply components or another upstream restriction. See irrigation supply and backflow repair.
Why replacing heads may not solve it
Changing sprinkler heads can mask a hydraulic problem without correcting its cause. Pressure diagnosis should start with where the loss occurs and whether it affects one zone or the entire system.
Fort Mill Irrigation has serviced residential irrigation systems since 1996. Call (803) 710-8008 for sprinkler troubleshooting.