Irrigation Controller Skips Zone Intermittently Decision Tree

Why this matters

A station that waters some days and skips others is the hardest irrigation fault to pin because the intermittency hides which of three things is failing: the solenoid (high resistance or marginal coil), the field wire path (a corroded splice or nicked wire arcing intermittently), or the controller station output (a marginal triac or relay). Techs replace solenoids that test fine, or condemn controllers that are healthy, because they test once, see it work, and move on. The right approach is a resistance and voltage method that distinguishes a coil problem from a wire problem from a board problem, measured at the controller terminals and at the valve, so you replace the one component that is actually marginal instead of guessing through three.

Symptom presentation

The customer reports a dry zone that runs "sometimes." The controller may advance through its program normally with no error, or it may post a station fault on the cycles it skips. The same station works on a manual test sometimes and fails other times, that intermittency is the signature. Adjacent zones run fine, which rules out a whole-controller power or common-wire problem. On smart controllers, the log may show the station energizing but the valve not opening (open-circuit fault) or a current draw outside the expected window.

Quick checks

• Read solenoid resistance at the controller. Disconnect the suspect station and common, measure ohms across them. A typical 24 VAC solenoid reads roughly 20 to 60 ohms. Infinite/open means a broken wire or dead coil; very low (near zero) means a shorted coil or wire-to-wire short.
• Read field voltage at the controller terminal during a call. With the station running, measure AC voltage station-to-common. Roughly 24 to 28 VAC means the board is delivering output; near zero means the station output is dead on that cycle.
• Wiggle-test the splices. At valve-box splices and the controller terminals, flex the wires while the station runs; an intermittent that comes and goes with movement is a bad splice or nicked conductor.
• Compare against a known-good station. Swap the suspect field wire to a known-good station terminal; if the fault follows the field wire, the controller is innocent.

Isolation tree

Branch A, solenoid coil marginal. Resistance reads borderline (very high but not open, or drifting on re-measure) and the field voltage is good. The coil pulls in some cycles and not others as it heats or the plunger sticks. The valve, not the controller or wire, is intermittent.

Branch B, field wire / splice intermittent. Resistance reads open or jumps when you flex the wiring; field voltage at the controller is good but does not reach the valve reliably. A corroded, water-intruded, or nicked splice arcs and breaks contact intermittently. This is the most common cause in older installs with non-waterproof connectors.

Branch C, controller station output marginal. Field voltage station-to-common drops below roughly 22 VAC or reads zero on the skipped cycles while a known-good station on the same controller reads full output, and the field wiring tests sound. The board's triac/relay for that station is failing.

Branch D, common-wire fault affecting one station path. Less common: a high-resistance common shared by several zones drops voltage only when that zone's solenoid adds load, manifesting as one zone being marginal. Reveals itself when several zones degrade together under combined draw.

Confirming diagnosis

Branch A: Measure the solenoid at the valve itself (not just the controller) to remove wire path from the reading; a marginal/drifting coil resistance at the valve with good voltage delivered confirms the solenoid. Swap in a known-good solenoid and re-run several cycles, the intermittency should clear. Branch B: With a meter on the station, flex each splice and the wire run; voltage that drops or chatters on movement confirms the connection. Re-strip and re-splice with waterproof connectors, then re-test. Branch C: On the skipped cycle, confirm the controller terminal reads near zero or low while a good station reads full output; moving the field wire to a spare/known-good terminal and seeing the zone run reliably confirms a bad station output. Branch D: Measure common-to-ground resistance and compare voltage drop across the common while multiple zones run; elevated drop indicts the common splice or run.

Remediation

Branch A: Replace the solenoid (or the valve if the bonnet/diaphragm is also compromised); these are inexpensive consumables and a marginal coil only worsens. Branch B: Cut out the suspect splice, re-strip clean copper, and reconnect with grease-filled waterproof wire connectors rated for direct burial; never use indoor wire nuts in a valve box. Re-test through several cycles. Branch C: Move the field wire to an unused station terminal and reprogram, or replace the controller if no spare stations remain; for modular controllers, swap the affected station module. Branch D: Repair the common splice or upsize/replace a degraded common run; a sound common restores all affected zones.

References

1. NFPA 70 National Electrical Code, Article 725, Class 2 low-voltage control circuits
2. Irrigation Association, Landscape Irrigation Technician troubleshooting reference
3. Hunter Industries / Rain Bird controller service manuals, station-output and solenoid resistance specifications
4. ASABE/ANSI S376, Design and Operation of Irrigation Systems
5. 3M / DBR/Y waterproof connector listings for direct-burial irrigation splices (manufacturer specification)