I have an 01 GP 3.8 n/a. It has a few issues that I want to get fixed so I can reliably drive it.

P0420 - Catalyst efficiency, bank 1 - Is there any way to know exactly what causes this code is other than just changing O2's, then the cat or vise versa? It does have an exhaust leak at the downpipe connection that I plan to get fixed tomorrow. Would that alone fix the code/dying all the time?

It also has oil in the anit-freeze. Do these cars have an oil cooler that could also be the problem or is it for sure a head gasket?

It also wont shift into overdrive. What are my options here?


Any help with these issues would be greatly appreciated! Thanks!