Help! My A4 can't or won't pass inspection

[Yogibara]

Greetings all,
I purchased a 2007 A4 2.0t Quattro for my son and am dealing with a couple of nagging problems. When I scanned for active/pending codes I got quite a few but was able to clear them except for the pesky P0420 code. Here are the details:
141k miles
Well serviced throughout his life (his name is Olaf; see CarFax (https://1drv.ms/w/s!Alp90rswbwYGhZo4nb9RhlMmlaVURA?e=4t6umR)), purchased for 5K. I didn't have my OBD scan tool so i purchased it the old fashioned way. I watched the video series by "Chris Fix" on how to inspect a used car. [:D]
These were the original codes (NO CEL)
P0190
P0420
P0087
P0087
P2294

I cleared the codes and ran the car again; P0420 was the only one that returned but still no CEL. I did experience a "limp mode" briefly but no code.
Troubleshooting steps and Timeline:

Inspect exhaust for leaks - none detected
Inspect/replace cam follower (why not, right? the car has 141K miles so I expected the worse) - some wear but no concaving or damage. I replaced it anyway. All codes cleared and did not return.
Attempted to have the car inspected and it failed. It seems (most plausible) the battery was replaced and the car hadn't completed the drive cycle.
Thinking that the car just needed to be "driven more" I let my my son drive it from Houston to Dallas but the CEL came back a week into him driving it. I purchased the BlueDriver OBD and had him scan for codes, smog test, Mode6 and freeze frame.
P0420
Smog Test failed - Catalyst Vehicle has not yet completed this test, Evaporative System Vehicle has not yet completed this test, Oxygen Sensor Heater Vehicle has not yet completed this test, Oxygen Sensor Vehicle has not yet completed this test
Replace both O2 sensors
I flew up to Dallas and drove the car back so I can repair.


Check Engine Light returned with P0420. Mode6 report shows:
Catalyst Monitor Bank 1
Manufacturer Defined Test
Status: Complete and Fail
MID: 21 Min: 4096 Val: 3053
TID: 84 Max: 65535

So before I throw another nickel at this car can anyone tell me what "Catalyst Monitor Bank 1" is?
I reviewed a live data stream of the downstream O2 sensor while driving it back from Dallas and observed a frequent "0.0" voltage reading throughout my trip. Is that normal behavior? See chart (https://docs.google.com/spreadsheets/d/1lOkev5svPpNiF4HCv3a60u8K5QuJFexYB4HjKroyPY4/edit?usp=sharing).

My gut is telling me that the catalytic converter is the culprit but I could really use another set of eyes on what I'm looking at. Thanks!

[Antekaudi]

P0420 is Catalytic Converter Efficiency code.

Most like your catalytic converter is done or missing.

I would do a visual inspection first. You can take out the primary O2 sensor and put a small inspection scope inside to inspect the cat.

There are ways to bypass the code but it all depends on your state emissions regulations and what they check for. I am sure you can find more info about that on the forum.

[devon1996]

141k miles is a lot on these cats the computer is super sensitive. Put a spark plug defouler on the 2nd 02 sensor and you might be able to fool it. I've had it work on an 09 before.

[Yogibara]

Thanks for the replies so far. This morning I tried running Cataclean through the system but only made things worse. The car did NOT like that stuff and stalled after about 15 minutes of heavy driving. It now sounds like a diesel engine as it burns it up. What's weird is that I needed jump to start the car. Oil pressure warning also popped up. *sigh*

Michael

[krewbrew]

Sorry for your headaches on this. I have a used cat from my 2008 A4 that had 80,000 miles on it if you would like to buy it.

[Yogibara]

Sorry for your headaches on this. I have a used cat from my 2008 A4 that had 80,000 miles on it if you would like to buy it.

I may take you up on that! PM inbound.