There are two cats and four sensors on the 3.0. I tried to do this replacement myself and gave up pretty quick when I found where all of them were located. The two pre-cat sensors are loacted just to the left (facing the vehicle) of the engine right after the manifold. The post-cat sensors are located behind the right front wheel - you can see them by crawling under the vehicle, but you probably need to remove the wheel and fender liner to get good access to them.

FYI, if you ever have to replace the cats, get ready to shell out some money. The cats are basically mounted to the manifold and there are no aftermarket solutions out there - cost me $2200!! My indy mechanic said that was the first and last one he will ever do - they actually had to loosen some of the motor mounts to access the manifold.

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2001 X5 3.0i (PP/WP/AP) Traded at 213K miles
RARE White on Black - Why does everyone get Tan??
First E53 with 215s on XOutpost!!