Zone won't turn on: coil → wiring → diaphragm
First, run the zone from the controller and put your ear near the valve — a healthy solenoid clicks. No click: swap the coil with a neighbouring valve's coil. If the fault follows the coil, buy a coil. If it doesn't, it's wiring — usually a corroded joint in the valve box, fixed with waterproof connectors.
Clicks but no water? The diaphragm inside the valve is stuck or torn — a rebuild kit or full valve swap is the fix.
Zone won't turn off: it's not the controller
A valve stuck on is almost always debris under the diaphragm (very common on bore water) or a torn diaphragm — not an electrical fault. Isolate the water, open the valve top, rinse, inspect, and rebuild or replace.
All zones dead: look upstream
When nothing runs, stop testing individual valves. Check the controller has power and the fuse is intact, then the common wire, then the master valve or pump-start relay if your system has one. On bore systems, confirm the bore itself is delivering — a dry bore mimics an electrical fault.
The parts you'll want on hand
Nine of ten solenoid faults are fixed with: a spare coil, a diaphragm/rebuild kit, and a handful of waterproof wire connectors. All three live on our shelves for exactly this reason.
Swapped the coil, checked the wires, and the zone still sulks? That's the point where a pro saves you money. Solenoid faults are Perth Reticulation Experts' bread and butter. Have PRE find the fault.