One Sprinkler Zone Runs With All Others: Master Valve or Wiring Fault?

When zone 2 activates alongside every other zone—even with its controller wire disconnected—the issue is either a master valve problem or crossed field wiring. Here's how to tell the difference.

Scenario A: System Has a Master Valve

Check the controller for a wire on the MV (master valve) terminal. If present, the master valve supplies water to all zones. A stuck-open master valve (usually due to a torn diaphragm) lets water flow to zone 2 whenever any zone runs. Since zone 2's own valve gets pressure, it opens. Fix: Rebuild or replace the master valve.

Scenario B: No Master Valve

If there's no MV wire, zone 2's valve has constant pressure. Disconnecting its controller wire should stop it—unless the hot wire for zone 2 is shorted to another zone's hot wire in the field. When zone 1 fires, voltage bleeds to zone 2's wire, activating its solenoid. Fix: Trace and repair the damaged wire splice.

Quick Test

1. Disconnect zone 2 wire at controller.
2. Run zone 1 manually.
3. If zone 2 still runs → master valve issue or wire short.
4. If zone 2 stays off → controller was the fault (unlikely here).

Most often, it's a master valve diaphragm or a nicked wire from landscaping work.