I just DIY replaced the cats. I would try O2 sensors first.
I was getting EGR flow low and catalyst below 90% check engine lights and codes.
2009 3.8 automatic, 130,000 miles. left side exhaust manifold cracked (again.) Emissions test due.
I used a $600ish set of cats from ebay. front bosch o2 sensors and rear mopar o2 sensors from rock auto. Those all fit just fine.
The Dorman branded, made in china exhaust manifold is the worst casting I've ever seen. #1 cylinder isn't even close to the shape of the gasket and port on the head. The chinese casting has threaded holes for the headpipe and a threaded hole and bolt instead of a stud for the heat shield. The included heat shield was 1 piece and about 1/2 the coverage compared to original. The previous Dorman manifold was fine and looked/fit exactly like the original. It also cracked exactly like the original.
Getting the old y pipes down wasn't too bad. There was just enough room to pull it down. Getting the replacement system back up was way more challenging. Sure it is in 2 pieces, so you would think it would be easier, but it was not. I ended up removing a front suspension link, and I had to drop the transmission crossmember (automatic 3.8). Lower the back of the transmission, pull the exhaust pipe rubber hangers off...Finally I got the driver's side back under there. All bolted up, the "Y" of the pipes hit or are way too close to the transmission mounts on the framerail. It just doesn't fit like the original and this is a game of millimeters of clearance. Also, with a traditional muffler clamp in the middle of the crossover pipe, you can't put the skid plate back on and there is no heat shield on this crossover pipe right in front of the transmission pan. Looks hot.
The pipes are louder, the holes in the catalyst matrix are larger, the exhaust is slightly louder too. Power seems to be the same.
the rig passed the IM240 emissions test no problems at all. CO was 3. something out of 15 max. HC and NOx were a long, long way from the maximums.
In retrospect, I might have been able to simply replace all 4 o2 sensors and solved the EGR flow low AND cat efficiency errors. Given the cost and pain in the ass factor of the catalytic converters, It would be worth trying new O2 sensors even if they are hard to reach.
Pull the inner fenders. It is worth it to get to the exhaust manifold to headpipe bolts.
Last edited by rudycon; 07-17-2018 at 09:55 AM.