P0420 code

[KawiZ4]

Hi all, I've tried to search but don't see much on the forum. When i bought the car it had the check engine which went off while i test drove. It's stayed off for a bit and came back on. Read with scanner and showed a P0420-Catalyst system, Efficiency below Threshold-Bank 1.
From what i read, this should be rear sensor (after cat) on bank 1- cyl 1-3?
I've heard of people removing it and testing with a multi meter to look for voltage about 1VDC, then a quick drop after heat is removed. Performance hasn't changed a bit, no issue at all other than the code. I just listed it for sale due to my current situation, and don't want to have a code /check engine taking away from what a great car this is. I can't imagine the cat is shot at 67k miles, and no issues/smell, so i'm hoping it's a bad 02 that i can replace.
Thanks all. (of course i just saw someone here had a full set for sale and they're gone!)

[MarinerZ4C]

Had same code on 2005 E46. Use a data-logging OBD II reader and monitor your O2 sensors (all four). Do a search on E46 Fanatics forum. Because it's a well-populated forum, you'll find a lot of discussion on some faults common to many BMW's. My issue was addressed with four new O2 sensors.

You're correct in that it could just be your downstream sensor(s), especially if you look at maintenance records and find that the previous owner replaced the upstream sensors for running issues but didn't replace the downstream at the same time (to save costs). If the downstream sensor is actually responding full scale but tracks the upstream sensor, it could indeed be catalyst malfunction. If that's the case, and the vehicle is under 100K miles, there may be warranty coverage. The E46 I owned had a cat/manifold replaced under warranty to the original owner at only 31K miles - so they can go bad, and if so, you want it fixed by BMW while they'll cover it.

[KawiZ4]

Great info, thank you. E46 threads were helpful as well.
i'm going to take a good look at connections, look for leaks, then maybe remove the 02 and test it with a torch/meter. from there i'll dig in further as you mentioned with OBD.