Regensensor

The optical sensor bonded to the inside of the windscreen that decides when the wipers run — and, on most cars, when the headlights come on.

It measures the glass, not the rain. Infrared is shone into the windscreen at an angle that makes it bounce back internally; water on the outside lets some of that light escape, and the drop in what returns is what triggers a wipe. Nothing ever touches the water.

Which makes the windscreen part of the sensor. It is bonded on with an optical gel pad, and a replacement screen, a reused or bubbled pad, a slightly misplaced sensor or an aftermarket glass with different coatings all change what comes back — so a rain sensor fault shortly after a screen replacement is a fitting problem until proven otherwise, and belongs back with whoever fitted it.

The everyday cause is simply the film on the inside of the glass, which the sensor has to look through and which builds up over years.

One consequence worth knowing beyond the wipers: the same sensor usually drives automatic headlights, so a car whose lights stop coming on in poor weather may be reporting a rain sensor fault rather than a lighting one.