Control Valve Won't Shut Off: Stuck Solenoid or Diaphragm

When a zone valve stays open after the controller turns it off—and tightening the bonnet doesn't help—the solenoid plunger is likely stuck in the "up" (open) position, or the diaphragm is torn.

Quick Diagnosis & Fix

1. Cycle the zone on/off repeatedly from the controller. Sometimes the magnetic cycling frees a stuck plunger.
2. Remove the solenoid (unscrews from bonnet). Inspect the plunger—it should move freely in/out. Clean with vinegar if mineralized. If it binds or the spring is weak, replace the solenoid ($15–$25).
3. If solenoid checks out, inspect the diaphragm. Remove bonnet, check for tears, holes, or debris preventing seal. Replace diaphragm if damaged ($10–$20).

Why Solenoid First?

Solenoid failure is the most common cause of "valve won't close." It's the easiest and cheapest part to replace. Diaphragm tears usually cause weeping (slow leak), not full-open failure.

Pro tip: When reinstalling the solenoid, don't overtighten—snug + ¼ turn. Overtightening distorts the plunger bore.