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I've never replaced one on a Nissan, but here's the general overview:
1. if the abs is coming on for no reason, then yes, one of the sensors thinks the wheel isn't turning any more. The sensors are generally long-lived and don't often fail.
2. it'll either be an optical sensor or a magnetic pickup.
3. If it's magnetic (looks like a bit of plastic), make sure it's positioned correctly. Normally there'll be a screw thread that will run the sensor in to about 1-2mm of a toothed disk. Any more than 1-2mm will cause the sensor to start failing (and hence think the wheel isn't moving). Of course, any less and you'll shear the head off it. If the positioning looks ok, then it's probably knackered. The way to set these is to run the sensor right up to the disk, then back it off 1/2turn (i.e. so that it's as close as possible without actually touching).
4. If it's optical, make sure the lens is clean! Some systems use a combined emitter/detector which shines onto a reflective strip. Make If this is the case, make sure the strip is clean. Other systems use a seperate emitter/detector shining through a toothed wheel. Again, make sure both lenses are clean. Again, change it if everything looks ok.
Testing these sensors is a bit of a bitch without the appropriate equipment. Magnetic sensors can sometimes be tested with a DVM set to resistance. Wave a piece of metal past them and look for a change in resistance. Problem is that it really depends on the DVM, since you'll actually be using the voltage from the DVM battery (it's a frig, but can work). optical sensors normally require a seperate power source and a DVM set to voltage.
Hope this helps, sorry it's a bit theoretical, it's come from an R&D background rather than in-service.
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