Greetings everyone. Well, I started this issue and wanted to give an update and ask for some advise. As mentioned, I was receiving CEL Code P0421 & then started receiving P0431 a few weeks later. I finally buckled down and had both the Cats replaced on my car by a local muffler shop. Ken's Mufflers in Dallas, TX. Great guy....been doing work for me for over 10 years and I trust him completely...always a quality job. I must admit, AudiTech79 if you're reading, that they did tell me they would NEVER do another one of these again due to the difficulty in changing these out. He had the car over a week (I wasn't in a rush anyway). They got the original parts replacement, did the repairs, basically put the car back together again (ha) and cleared the code.
And the story continues.....I've been driving for about a month now. No issues. No lights. Finally happy with this second hand purchase. Well, until today! Guess what came back on: CEL0421 & CEL0431. Now that I've had the Cat's replaced, can anyone provide further troubleshooting advise. I'm honestly hoping it's a simple O2 sensor or something. Even after spending the money for this quite expensive repair, I just want the darn thing fixed. As I know weather can sometimes affect a car's performance, the temperature has been swinging 50 degrees (between 20's & 70's the last few weeks here). I do let it run for a minute or more every cold morning just to get the chill off. The light came back on today driving down the freeway at about 60mph in normal traffic. No stopping & starting.
Posts: 4995
Joined: 2/24/2007 From: Sacramento, CA Status: offline
Its a software issue, if you have any engine except the 3.0L then go to the dealer and tell them that you have brand new cats installed but its still throwing a cat efficiency code. They will know what to do. If it is a 3.0L, well i don't know what to tell you.....
Figures....I have the 3.0. I'm just going to put this car on the side of the road and let someone steal it. Appreciate all the advise and expertise. Have a good evening.
Mr Auditech, Why won't the dealer know what to do with the 3.0? Its still an audi motor that's been around for years.
I'm looking at buying an 02-04 a6 3.0 an am leaning toward the 3.0 because its non turbo, thinking its less complex to work on and more reliable. I have heard that I won't have to worry about the coils going out and the turbos failing.
Well I'll tell ya', honestly the car is a good size and if it weren't for this flippin' light I'd enjoy it. Funny thing is that Audi said they were going to replace the Cats too so that's what I had done. At this point, I don't know if I would even trust the Audi dealership. Best of luck if you buy one. Mine's an 2003 A6 with brand new Cats and it's still having problems. I was inclined to get the O2 sensors replaced next but don't know what to do now.
Dallas, I'm no audi mechanic, but here's my thoughts on this. Maybe your aftermarket cats aren't up to the spec of the audi units and aren't removing enough of the bad emissions, so they are still tripping the 02 sensors and making them throw the codes. I hope that's not the case since you just spent big bucks on them.
Does the car run fine?
There is some diagnostic software that comes highly recommended called Vag-com. Check it out at: http://www.ross-tech.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=VAGCOM I hear its better than the diagnostic tools that Audi supplies to their dealers. I think it can clear codes and it gives descriptions of what your codes mean so you don't have to go look them up. You should install the software on a laptop and put it in the car. It hooks into the cars computer with a special cable.
Its worth checking out. I plan on buying it. Good luck.
PS Please let all of know what kind of a solution you come up with for this problem.
I got a new cat for a 96 A6 for about $600 from the guys at Force Five in Concord NH. 603-223-6700.
Sorry to sound like a broken record - but Chris at Force Five has been a big help. Without his low prices on new parts and the ability to get used ones from him, I would have been forced to switch to fords or some other bottom feeder like a Honda, Toyota, or Lexus.
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1997 A6 Avant 1998 Jetta 2007 Dodge Grand Caravan SXT one High Performance Roller Skate
On my A6 Avant, I have been getting these same codes since at least 80K in 2004. P0421 (Pri Cat Bank 1), P0431 (Pri Cat Bank 2), and P1423 (secondary air flow too low bank 1) always show up 1 or 2 days after being cleared.
I can guarandamtee you these cats did not go bad at 80K.
How to know? I always knew those emissions tests in Denver would be good for something. In 2004 at 66K, my hydrocarbons were 0.07 gm/mi and NOx was 0.2 gm/mi. In 2006, at 136K HC were 0.13 gm/mi an NOx 0.3 gm/mi. Pass/fail cutpoints are 1.2 for HC and 3.0 for NOx. This car is pretty darn clean from an emissions standpoint.
If the cat was shot, HC would be much higher, trust me, people I know who have had bad cats have failed the emissions test hands down. Even if my cat is bad, why would I replace it if my car runs that clean? That makes no sense.
I need another emissions test this year (177K) so will do that before taking to my mechanic this week. I will try to have him do a pre- post- cat temperature reading.
The fuel economy on this car has always ranged from 20-22 mpg, so I don't believe it is an O2 sensor. I believe it is a secondary air leak issue that is of very minor concern (except for that damn CEL/MIL). Hope to have a diagnostic on that this week.
I love your gurantee about the cats at 80K. If you really believe that I'll give you a gurantee that the land I'm selling 2 miles east of Miami is dry.
I believe your real problem is the secondary air pump. It usually takes a few days for it show up after being reset. The cat errors are probably coming along for the ride -- but you might be seeing a sensor problem.
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1997 A6 Avant 1998 Jetta 2007 Dodge Grand Caravan SXT one High Performance Roller Skate
Printable Version
RE: CEL: Code P0421 - 
RE: CEL: Code P0421 - 
- but Chris at Force Five has been a big help. 
Without his low prices on new parts and the ability to get used ones from him, I would have been forced to switch to fords or some other bottom feeder like a Honda, Toyota, or Lexus. 
). Hope to have a diagnostic on that this week. 
