One of the best reasons to change this part before it fails occurs when the plastic sheath in its cable cracks. Because the sensor is installed at the top of the tone ring or flywheel it monitors, there's no way to form a drip loop; any water that enters the cable sheath will wind up inside the sensor, rust the magnet core and swell it into the fragile alloy bracket. Most of the time the sensor will keep doing its thing, not calling attention to itself.

One reason NOT to change this part in preventive mode is the incidence of out-of-box failures is high as reported many times in the forums. The fine wire and epoxy encapsulation combine to make this sensor a difficult manufacture. And you can imagine, those making it are doing short runs compared with when it was used in production cars.

So, look at the cable sheath at least, to judge the one you have. If it is broken, do change the sensor as soon as you can. My experience says there's no advantage to buying the "OEM" brand (Electricfil/Bougicord) and my skeptical nature suggests the replacements are all made on the same line in China now. Volvo is not doing rigorous incoming inspection on parts for cars 30 years out of production.

--
Art Benstein near Baltimore

"There are three kinds of men: The ones that learn by reading. The few who learn by observation. The rest of them have to pee on the electric fence and find out for themselves."