A rain sensor fails in one of two directions, and they look nothing alike. Failed open, it stops interrupting the cycle and the system waters straight through a storm, which is invisible until a bill arrives. Failed closed or saturated, it holds every zone off indefinitely, which looks exactly like a dead system. Most controllers have a sensor bypass switch, and flipping it separates a sensor fault from everything else in seconds.
How a rain sensor works
A rain sensor sits in the sensor circuit between the controller and the valve common path, and it acts as a switch. In dry conditions it is closed, letting the circuit complete so zones can run. When it detects rainfall past a set threshold it opens, breaking the circuit, and no zone can be energised regardless of what the program says.
The common type uses a stack of hygroscopic discs that swell as they absorb water and physically open a switch, then shrink as they dry and close it again. So the sensor is not measuring rain in real time; it is measuring how wet its own discs are, which is a proxy for how recently and how heavily it rained.
That mechanism explains both failure modes and the most common complaint about them: a sensor that keeps the system off for days after a storm is not malfunctioning, it is drying slowly.
The two failure directions
Worth noting that the second row is a genuinely common cause of "my whole sprinkler system has died". Nothing is broken, nothing needs digging, and the diagnosis takes one flip of a switch.
| Symptom | Cause |
|---|---|
| System waters during and after rain | Sensor failed open, disconnected, wired out, or bypass switch left on |
| No zone runs at all, controller looks normal | Sensor failed closed, saturated, or the sensor circuit is broken |
| System off for days after rain | Usually normal — discs drying slowly. Check siting and threshold |
| Intermittent — works some weeks, not others | Marginal sensor, damaged wiring, or a failing wireless link |
| Never seems to interrupt anything | Sensor mounted where rain cannot reach it, or threshold set too high |
| Wireless sensor unresponsive | Dead battery, lost pairing, or out of range |
What to check yourself
- Find the bypass switch. Most controllers have a sensor bypass or sensor active switch. Set it to bypass and start a zone. If the zone now runs, the sensor circuit was holding the system off.
- Look at the sensor. It should be mounted in the open where rain can reach it — typically on a fascia, gutter or post, away from overhanging eaves and tree canopy. A sensor under an eave never gets wet, which is why it never interrupts anything.
- Check it for debris and nests. Leaf litter, pine needles, spider webs and insect nests all interfere with the disc stack or the switch.
- Check the battery on a wireless unit. A dead battery in a wireless sensor can leave the controller either ignoring it entirely or treating it as tripped, depending on the design.
- Note whether the wiring is intact. A visible break or a chewed cable in the sensor wire is a plausible cause of either failure direction.
- Leave the bypass alone afterwards. If you switched to bypass to test, remember it. A bypass left on all season means the sensor is doing nothing, and that is one of the most common ways a working sensor ends up useless.
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Siting is usually the real problem
When a sensor has never worked properly, mounting is the first suspect rather than the sensor itself. It needs to be in the open, above the spray pattern of the irrigation itself, clear of eaves and canopy, and somewhere it will actually dry out afterwards.
Mounted under an eave, it stays dry through every storm and interrupts nothing. Mounted inside the reach of a sprinkler head, it trips every time the system runs and then holds the system off — which produces an entertainingly self-defeating loop. Mounted in dense shade, it takes days to dry and the system stays off far longer than intended.
The threshold matters too. Most sensors have an adjustable trip point, and setting it sensibly for local rainfall patterns is part of a proper installation rather than an afterthought.
Sensor or smart controller?
A rain sensor answers one question: has it rained recently enough to skip watering? A weather-based smart controller answers a more useful one: given recent rainfall, temperature and evapotranspiration, how much water does this landscape actually need right now?
For North Georgia’s pattern of heavy, localised summer thunderstorms, that difference is real. A sensor knows it rained; it does not know whether it rained a tenth of an inch or two inches. Weather-responsive scheduling accounts for the amount.
That said, a properly sited and functioning rain sensor is inexpensive and does most of the work for a conventional system, and many smart controllers still support one as a local backstop for when the weather feed and reality disagree. As a general North Georgia market figure, rain sensor replacement commonly runs roughly $95 to $225 installed — a market estimate, not a quote.
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