Engine Performance problem
2004 BMW 320 4 cyl Two Wheel Drive Manual 107000 miles

hi
i drive a 320d that has recently had its turbo done by a professional bmw outlet. after that the performance was brilliant.
now an exhuast place cut out the second catalyst on the exhaust and replaced it with a normal box. now all of a sudden the performance is very bad.

are they related. what does the second catalist do?

The degradation in performance might be attributed to the quality of the exhaust components used in the work performed by the exhaust place. What was wrong with the exhaust system, was it stock, was this an elective or compulsory enhancement?