How a Rain / Freeze Sensor Actually Talks to the Controller

A rain/freeze sensor does not "decide" anything on its own — it is a passive switch that opens or closes a low-voltage loop, and the controller is the device that interprets the loop state and decides whether to start a zone. Most residential and light-commercial sensors (Hunter Mini-Clik, Rain Bird RSD, Toro RainSensor, and the new smart RF models like Hunter WR2 and Rain Bird WR2 RFC) wire into a dedicated pair of sensor input terminals on the controller that sit in series with the common return on each zone valve. The sensor head closes a small internal contact when it is dry (the "normally closed" loop) so the controller sees a closed circuit and is free to run; when the hygroscopic disc swells or the freeze thermistor drops below its threshold (~37 F / 3 C on most rain-freeze models), the contact opens, the controller senses the open loop, and it suspends scheduled watering. Two things to keep straight when you read the rest of this page: bypass on the controller's user menu is a software flag that flips the sensor input state internally (so the controller pretends the loop is closed), while a jumper wire is a physical short across the sensor input terminals that electrically bypasses the sensor head; these are not interchangeable and using the wrong one is the most common reason bypass "doesn't work." Finally, the smart RF sensor variants still terminate at the same controller input pair — they communicated wirelessly to a receiver module mounted at the controller, but they still close/open the same loop electrically.

The Four Symptom Sets — What Each One Rules Out Up Front

Before you touch a multimeter, sort the symptom into one of four categories. They map directly to the four most common failure modes and they will save you an hour of chasing the wrong cause. (1) False-tripping system — the sensor stops watering on clear, dry days, often in the morning when dew is still evaporating. Rules out: wiring issues, controller firmware, relay failure. Points to: sensor-head condition, overspray, mounting, residual moisture in the disc. (2) No shutdown in rain — the system runs through a thunderstorm, every head popping up while water falls on the sensor head. Rules out: bypass-on or missing jumper. Points to: stuck-relay or welded contacts, corroded terminal, broken jumper, saturated disc past reset. (3) Bypass mode not honored — you set the bypass switch, the controller display shows "sensor bypassed," but the system still won't run, or it continues to ignore a sensor that is clearly dry. Rules out: head/sensor condition. Points to: controller-side sensor input wiring, jumper-cable vs. software-bypass confusion, common-terminal swap on smart controllers. (4) Irrigation running in freezing weather — heads pop up while the temperature is in the high 20s or low 30s and remain coming up across a deep cold snap. Rules out: rain-only symptoms, physical loop failures (those would also stop watering in clear weather). Points to: a rain-only model installed where rain-freeze is required, the freeze threshold not user-adjustable on this sensor, sensor on the wrong terminal.

Multimeter and Wire-Continuity Test at the Sensor

Pull the controller's transformer before opening anything — the sensor loop carries 24 VAC on most controllers; live testing is safe for the controller but a probe slip on a charged terminal is dangerous for you.
Locate the two sensor terminals on the controller (typically labeled "Sensor," "SENS," or paired with a small jumper tab across them). Note any jumper present.
Set your multimeter to continuity (beep mode) or resistance (ohms).
Place one probe on each sensor terminal. With the sensor head in a DRY state (discs not swollen, ambient above the freeze threshold), expect a closed loop: 0–2 ohms, multimeter beeps continuously. A reading above 5 ohms indicates a corroded terminal or a marginal contact.
Now wet the sensor disc with a spray bottle (or hold an ice pack against the thermistor for rain-freeze models) and watch the reading. Expect the loop to open: infinity (OL), multimeter silent. A reading still showing continuity when the sensor is wet means the contact is welded closed — a hard sensor failure.
Reverse and repeat: dry the disc thoroughly (paper towel + 10 minutes of vent time) and re-check — the loop must close again. If it does not close after 10 minutes, the disc is saturated past reset or the vent slots are blocked.
If the wires run through a junction box in the ground, open the box and check the wire nuts — they're a frequent failure point. Pull each nut, look for green corrosion, re-strip and re-make the splice with fresh waterproof connectors.

Sensor-Head Inspection Walk-Through

Most false-tripping problems are visible to the eye before they show up on a meter. With the sensor unmounted (or at least with line of sight to the cap), check the following in order. Cap and disc condition: the cap should spin freely on the vent slots and the disc underneath should be smooth, slightly translucent cork-colored when dry, and uniformly black when wet. A disc that has gone chalky-white has UV-damaged the hygroscopic coating — no amount of drying will restore it; replace the disc. Mounting height: the cap must sit above the highest sprinkler spray arc on the property. The most common mounting mistake is to put the sensor on the fascia or under the eave where it sees overhead droplets from a nearby rotor — that rotor's mist is enough to swell the disc and trip the sensor on a clear day. Overspray path: walk the sprinkler zones with the controller in manual mode and watch every zone in turn. Any stream that reaches within 4 feet of the sensor cap is the cause of the false-tripping — relocate either the head or the sensor. Debris on the cap: leaves, pollen, cottonwood seeds, spider webs, and bird droppings on the vent slots block drying. Blow them off with compressed air (low pressure only — high pressure can crack the cap), then test. Orientation: the vent slots should face downward or sideways, never straight up. Sensors installed with vents facing up collect rainwater in the disc chamber and stay wet long after the rain has stopped.

False-Tripping Decision Tree

Phantom rain (no actual rainfall recorded): check the irrigation zones — is one head's overspray reaching the sensor? The most common culprit is a rotor installed within 10 feet of the sensor, throwing a mist that triggers the disc at dawn when dew is also high. Fix by either moving the head, redirecting the nozzle, or relocating the sensor to a position higher than the spray arc.
Overspray from a specific zone: with the controller in manual mode, walk each zone one at a time and watch the sensor from 30 feet away. The zone that triggers the disc during manual operation is the offending zone.
Condensation under the cap at dawn: this is a vent-slot orientation problem. If the cap is mounted with slots facing up, overnight cooling forms condensation inside the chamber and the disc stays wet until the morning sun warms it. Re-orient with slots facing down or sideways.
Post-storm residual moisture past manufacturer's reset time: the disc has dried to within tolerance but the hygroscopic material has absorbed enough residual moisture to keep the contact marginally open. Most manufacturers spec a 4-hour reset time in dry air; a sensor that stays tripped 12 hours after a storm has either a saturated disc (replace) or a degraded vent slot (clean with a soft brush and compressed air).
Sensor dish is bowed or cracked: the cap or the cap-to-disc seal is physically damaged and moisture is leaking past the seal. Replace the cap or the disc; do not attempt to re-seal with silicone.

No-Shutdown-in-Rain Decision Tree

Hygroscopic disc saturated past reset: a real rain event but the sensor still says "dry" 30 minutes after the storm. The disc has absorbed so much moisture that the swelling force no longer overcomes the spring that closes the contact. Replace the disc.
Stuck-relay contacts welded closed: a power surge (lightning or utility brownout) welded the internal relay contacts. Multimeter shows 0 ohms with the sensor wet (see Repair step 5). Replace the sensor head — the relay is not user-serviceable on any consumer model.
Corroded terminal: green or white oxide on the screw terminal at the sensor head OR at the controller's sensor input. Resistance reads 5+ ohms when dry (should be 0–2). Clean the terminal with a small brass-bristle brush, re-strip the wire, re-terminate.
Broken jumper: a physical jumper wire across the sensor input on the controller has been left in place from a previous service call. The jumper electrically bypasses the sensor — the controller sees a closed loop whether the sensor head is wet or dry. Remove the jumper if a sensor head is installed and working. If you intended the bypass, set the controller's software bypass instead.
Spliced wire failure in the irrigation trench: a wire nut or Direct-Burial splice in the run between the sensor and the controller has wicked moisture and corroded internally. Splice may LOOK fine on the outside but the copper underneath is green. Cut back 6 inches, make a new splice with fresh waterproof connectors.

Bypass-Mode-Not-Honored Decision Tree

Controller-side sensor input is set OFF in programming: most modern controllers have a sensor input enable/disable toggle in the setup menu (sometimes called "Sensor Enable," "Smart Bypass," or "Rain Sensor On/Off"). The toggle defaults to ON; if someone disabled it in a previous service call, the controller will ignore the sensor regardless of bypass setting. Re-enable.
Jumper cable vs. software bypass confusion: a physical jumper across the sensor input on the controller ALWAYS bypasses the sensor — there is no way for the controller to "see" the sensor when the loop is shorted. If you want to bypass for a single watering, use the software bypass (a button on the front panel or a menu item); to install a permanent sensor, REMOVE the jumper.
Common terminal swap on smart controllers: smart Wi-Fi controllers (Rain Bird LNK / ESP-LXM, Hunter Hydrawise HPC, Rachio) sometimes share a common terminal between the sensor input and a smart feature (weather skip, flow sensor). If the sensor wires are on a "smart" terminal reserved for a different feature, the loop is closed by the controller itself and the sensor is irrelevant. Move the sensor wires to the dedicated sensor terminals called out in the controller's wiring diagram.
Bypass active but controller still shows "sensor active" on display: this is a known cosmetic bug on some controllers but functional bypass IS active — the zones will run regardless. Do not chase the display status; trigger a zone manually to confirm.

Irrigation-Running-in-Freezing-Weather Decision Tree

Rain-only model installed where rain-freeze is required: a Mini-Clik (rain only) does NOT have a freeze thermistor. In sub-40 F weather the irrigation will run because the rain sensor sees "dry" — the storm did not pass yet. Replace with a Mini-Clik II, Rain Bird WR2 RFC, or any sensor marked "rain/freeze" if your climate requires freeze shutdown.
Freeze threshold setting not user-adjustable on this model: some consumer rain/freeze sensors set the freeze trip internally at ~37 F (3 C) and do not allow adjustment. If your turf and shrubs can tolerate colder temperatures before you need shutdown (e.g., you are in zone 8 and want to water down to 28 F), this sensor will over-trip. There is no fix on the sensor; either accept the threshold or move to a smart controller with weather-based skip logic.
Sensor installed on the wrong terminal: a sensor that has both rain AND freeze capability MUST be wired to a sensor input on the controller that supports freeze skip. Some legacy controllers (pre-2010) ignore the freeze leg of the input. Verify the controller manual lists "sensor input w/ freeze" or "rain/freeze" support.
Freeze trip is intermittent: the thermistor is borderline — it trips one cold night but not the next, then trips again. This is the classic sign of a thermistor failing open. The sensor head is at end-of-life; replace.
Smart-controller weather data overriding local sensor: on a smart Wi-Fi controller, the local rain/freeze sensor is sometimes supplemented by an internet weather feed. If the controller's weather feed says "no freeze expected" (because the API feed is stale), it can override the local sensor temporarily. Disable the weather override or update the weather-source configuration.

Repair vs. Replace Criteria

Repair (disc replacement, cleaning, recalibration, wiring repair) if the sensor head is under 10 years old, the disc is the only failed component, no internal relay failure is detected on the multimeter test (Resistance stays at infinity under wet conditions), and the housing/cap are physically intact.
Replace the sensor head if the multimeter shows continuity when the disc is wet (welded contacts), the housing has UV-degraded past cosmetic damage (chalking, hairline cracks on cap), the thermistor is failing intermittently in cold weather, or the OEM disc kit is no longer manufactured for this model (Mini-Clik original style disc is NLA on most units built before 2010; Mini-Clik II disc fits those).
Replace the controller is almost never necessary — the sensor loop is so simple that controller-side sensor input failures are rare. A controller that "sees" a closed loop with no sensor wired (jumpers removed) is failing, but in practice this is far more often a miswired common terminal than a dead controller.
Document the model and the wire count on every service call: the sensor model + the wire color code at the controller matters for the next service visit. A photo of the controller's sensor terminal with the wires in place is the single most useful repair artifact.