Probably the back box. It is always the first to corrode as it runs the
coolest. The cat and middle box should be ok. My last E34 had 362,000miles
on it and it had only ever had one replacement middle box in all that time.
Then again it was mostly motorway miles so the whole system was nice and
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.