cdn A6 4.2

I got to take a little work-related trip today in the A6 (I opted to drive my own car instead of one of the Chevy Uplander company vans), and on the way home, I throw a CEL. VAG-COM tells me its the 16806 code again...main catalyst bank 1 efficiency below threshold. I had the same code back in February, cleared it, and now it has returned (big surprise there). I STFA, and there was very little on this code for the A6 (mostly TT's). Outside of that, it appears this code generally shows up with some other codes, but its the only code I'm getting. The solutions seem to be mixed...some a replacement cat and others O2 sensors. I did see a few posts by CarbonLord on this in the archives, but it appears the dealer just reset his code and that was it. If you read this CL...was that the end of the problem, or was there a follow-up visit?

My '00 only has 56k on it, so it should still be under the 8/80k emissions warranty if its the cat. Although the car is running just fine, I have a PA state inspection/emissions test coming up next month, so I gotta get this fixed soon.

carbonLORD

...over the course of that time, I had my left cat converter replaced, the maf sensor, the 0² sensor as well as the ABS sensor and a replacement air filter that crumbled and got stuk up in there, plus my vaculme lines were also replaced. I had CEL's for each one of these instances so its difficult to trace down the source from that 16806 code I realize, but my money is on the sensors, they all seem to fail at some point. Then again, theyve replaced my transmission so at this point anything is bound to go wrong and throw a code. Good luck with it, I wish I had a more definitive answer for you.

dave240Z

Given your mileage this is a distinct possibility. They are not terribly difficult to replace and are not terribly expensive either, relatively speaking of course so it's worth a shot. If you want to play detective, you could swap the O2 sensor from L to R and see if the problem follows. The difference in p/n on the various sensors is due to the wire length.

The problem with these little buggers is that they are vital for the engine emissions/performance and they degrade over time. For OBD II cars, they should be considered a wear item IMO. BMW actually recommends replacing them every 50K miles for "optimum performance". The reasoning behind this is that they don't usually throw a code until they are totally broken. This is bad because while it was degrading, the increasing inaccuracy leads to a richer fuel condition (while still within an acceptable range), which in turn destroys the catalytic converter, thereby beginning the downward and costly spiral.

Modern cars require 2 O2 sensors per converter, 1 pre and 1 post. The pre cat sensor is the one primarily responsible for the fuel delivery of the engine, whereas the post cat is basically there to verify that the converter is working properly. The pre cat sensor usually fails first and is the one primarily responsible for destroying the cat if it fails.

Converters by themselves are a passive part, so failures in the converter are usually the result of outside influences such as a rich fuel condition, which heats up the converter beyond it's normal range and effectively melts the small cores.

cdn A6 4.2

If they're OK, I'm off to the dealer for my free cat!