I had a clogged cat that caused my 7 to intermittently fall on it's face. The more power I gave it, the slower the car actually went. It got to the point finally where I couldn't get the car to even run. Mind you, this all happened within a day, not over months and months of time. I had the car towed to the nearest shop (my house was too far) and was gonna let them run their code reader...misfires on every cylinder, cat and 02 codes across the board. Not telling them what I did for a living, I left them car with them to charge the battery (slowly died from continual restarts/dying) and entertained them by letting them look over the car and they came back with a $4000 repair bill...wanting to do fuel pressure tests, remove the heads and check for carbon deposits causing the valves to stick, flow testing and cleaning the injectors, etc. Holding in my laughter, I got my keys back to find out they had broken my hood release. I went from laugher to being pissed and they claimed that my hood was jammed...odd, NEVER had an issue before. Their "free inspection" turned into a "we charge for the extensive looking over we gave your car", so told them we'll call it even for breaking my hood release, and they agreed.

So after running through in my head the codes I had, knowing I've already replaced one cat due to rattling, my last suspect was the remaining cat. I didn't suspect it since it was a factory replacement under BMW before it hit the 100k mark because they we're known to fail prematurely on the M62, which is the same motor in your X. While still in the indy's parking lot, I limped the car onto driveup ramps and removed the pre-cat 02 sensor. Car fired right up and purred like a kitten, mind you a loud ass kitten, but still purred. I removed the 02 sensor from the harness and drove to the nearest exhaust shop with a 1" hole in my exhaust...you'd be surprised how loud that can be! $200 (incl. installation) and I had my second Magnaflow flow cat installed and not a problem since.

Moral of the story...climb under your car, beat on the cats a few times to check for rattles, and go ahead and drive the car with the pre cat removed. Try doing one at a time, and if it gets better on either side, there is your culprit. I've also told you once before, after 100k you should probably replace both pre-cat 02's regardless due to them being wear items just like spark plugs. If in fact the car runs better with the 02's removed, I'd honestly replace the cats in a pair as well The piping is 2.25 and I think the model number was 94005. As a plus, you'll get a deeper exhaust note and regain some lost power if it's in fact clogged cats.

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