Quick answer: Irrigation wiring can fail because of damaged underground splices, corrosion, landscaping or digging, animal damage, aging insulation, improper previous repairs, or physical damage to buried cable.
Underground splices are common failure points
Irrigation systems rely on low-voltage connections in damp outdoor environments. Poorly sealed or aging splices can corrode and create resistance or an open circuit.
Landscape changes can damage wire
Edging, planting, fence work, drainage work and other digging can cut or nick shallow control wire. Damage may cause immediate failure or intermittent problems later.
Common-wire failures affect multiple zones
Several valves can share a common return conductor. Damage to that common path may disable a group of zones even though their individual zone wires are intact.
For diagnosis, Waxhaw Irrigation provides wire tracing and valve locating.
