If you tried to turn the car over with a key that did not have the immobilizer chip, the immobilizer would activate and the car would not crank at all. Easy indicator for the CPS, is if it doesn't want to turn over when warm, but turns over fine when cold. The sensors tend to get finicky when warm before failing completely.
It's not a difficult DIY, but can be a little annoying reaching up through the rear suspension. You can find more detailed info with a google search, but Pelican has a little info: (purple arrow in picture 4):
Porsche Boxster Engine Sensor Replacement - 986 / 987 (1997-08) - Pelican Parts Technical Article
Porscha
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