BMW Repair: 98 M3 Catalytic Converter, exhaust hangers, tulleys

Question
Hi Paul

Apparently I'm in the market for a new catalytic converter for my 98 m3 sedan with 120k miles on it.  I lost one of those rubber exhaust hangers that keeps your exhaust from bouncing around for a while.  Put a new one on and still had a rattle I couldn't account for.  Also got my check engine light turning on.  I'd say my performance has been a little off in the car too lately (like for a couple of months).  Since then (about 2 weeks since I put a new hanger on there) my rattle has maybe gotten louder and my light no longer comes on.  

I took it into Tulleys here in Nashua today to read the codes and fix my rattle.  They tacked down my heat sheilds and my rattle is gone.  They're saying the my codes are telling me that the catalytic converter is going.  It's $1850 and that price probably doesn't include new O2 sensors.

http://members.aol.com/SportsCarConvert/BMW35.html

has one that says it would fit my car for $405, and

http://www.ultimategarage.com/catalyst.html

says they've got one for $450.  Much cheaper.  I first of all don't have any problems spending the money if I'm getting a super product, but at over 3x the price, I find it to be not worth it.  I don't race, but do drive lots of long high speed highway miles.  In fact, soon, I'll be driving back to the Kansas City area.

So let me ask you this.  My light is not on.  My rattle is gone.  My performance still feels a little soft to me, but no other complaints.  How do I know for sure what needs to be done?   Can I just wait and see another O2 sensor light up and decide what to do then?  A friend who is pretty knowlegable tells me that I really should change the converter before my trip home as the backpressure build up can get bad enough to leave you stuck on the road.

what do you think I should do?

Thanks!

-Toby


Answer
most likely, the converter is breaking up internally. do you notice a slight rattle under the car when accelerating? if the catalytic converter is breaking up inside, it can set a check engine light. did the shop tell you what the fault said? it should have said something along the lines of catalytic converter efficiency. have you done a lot of driving since the light has gone off? sometimes it can take a while for a fault to reoccur. when you purchase a new converter, make sure it is a direct fit and is specific for an M-3. M-3 exhaust system is different than a regular 3-series. for $1850 you could purchase a complete exhaust. the choice is yours however.