Possible Solution for Catalytic Converter fault codes - P0421 & P0431
 

I was told something interesting that may save some of you a lot of money. I have a 2002 A4 3.0Q that I took into my local dealer (Cutrubus Audi/VW) for a CEL. I already knew what the fault was because I had it read at an Autozone where my brother works. It was a bad Cat and the code was P0421. I took it to my dealer this morning to take advantage of the federal warranty of 8 years/80,000 miles on emissions components so I could save money. As it turns out, the warranty applies to the in-service-date of the car which, for mine, was in February so the warranty expired ten months ago. When they ran the diagnostic they got two fault codes, one for each cat, but the readout said that each cat was below efficiency INTERMITTENTLY instead of being constant. When the service adviser saw this, he said that before I shell out $2000 for the repairs, I should buy an additive called '44K' and run it through the next two tanks of fuel. He gave me this speech about fuel quality and that I should only run 91 octane or higher and that I should only get it from an Audi-approved list of fill stations (i.e. Chevron, Conoco, Phillips 66, Texaco, etc.). I always run 91 octane or higher in all of my cars but I do buy it from a cheap supplier. Apparently, buying high octane fuel from 'lesser' brands/companies puts my car at risk for deposits and carbon build-up because of a "lower concentration of detergent additive in their gasoline" according to a letter AoA had sent to my dealer. My CEL could be what Audi calls a "false positive" meaning the computer thinks something is wrong when there isn't. The 44K additive is supposed to clean the gunk out and the CEL shouldn't come back. Don't know if anyone else has done this but I'm going to try it. I'll keep you guys posted.

Update: erratic fuel gauge behavior
 

So later today, I noticed my fuel gauge was acting strange. The dealer cleared my CEL before I left but this time it came back on after driving less than 15 miles. When I drove out of the dealer lot, my gauge indicated a full tank and the range computer said I had 400 miles until empty. After driving approximately ten miles the fuel gauge showed a decrease of a quarter tank of gas and the range computer reads 305 miles to empty. Could these issues also be caused by poor fuel quality?

yes.... poor quality fuels may have a higher concentration of sulpher and this builds up on the sender unit in the gas tank and gives wonky fuel level readings. i have seen this in both audi and vw.
i think what your dealer was referring to is "Top Tier" fuels, which are certified to be of a certain quality level determined by audi/vw and some other manufacturers

http://www.toptiergas.com/

i think they are right, get your min 91 oct fuel from one of the companies listed here

http://www.toptiergas.com/retailers.html

for the fuel level indicator issue, try adding a bottle or two of techron in each of your next couple tanks of fuel and see if it helps (i have heard it does help some ppl)

Update after adding BG 44-K
 

I added a can of 44-K to a full tank of fuel yesterday and drove about 200 miles. The CEL has not cleared itself though I doubt it will anyway; I think it has to be done by computer. I have noticed that my MPG is starting to improve, but it could just be in my head.

had the same issue, had a friend at an auto shop clear the light and have used 91/93 octane from sunoco ever since (not my first choice but the best option around) CEL never came back on 9 months and still running strong.

or alternatively you can run ~ 2 cups of acetone per each 20g in the fuel tank and it should have a "cleaning" effect on your O2 sensors. (I wouldn't do it) i will just put in a new cat and be done with it. 6 bolts to wrestle,l 2 O2 sensors R&R and in it goes. Going with the flea-bay lifetime warranty direct fit replacement..... any one used them before????

oh and before anyone starts trying to "educate" me on the "you get what you pay for" subject. There are only 2-3 washcoat suppliers in the world, and all have comparable quality control. I should know this as i used to design catalytic converters for an oem in the past.

The fault will come back. They always do. 6 minutes or 6 months... doesn't matter.

So I guess what they say is true...
 

If your Check Engine Light isn't on, you're not driving an Audi.

had the same codes. 1st time the dealer cleared and everything went back to normal. Car never ran any differently. 2nd time codes cleared after about 4 days. Original issue was about 6 months ago. So far so good.

So what did you do to fix it, anything? What did the dealer say about it? I need to run my second can of BG 44-K and then have the light cleared to see if it did the trick. Just out of frustration, I went and drove an 02 Lexus GS430 to see if it would make a good replacement for my A4. There's no doubt it would be more reliable than the Audi but it was just so sedate and boring. Afterwards, driving home I could really tell how much fun the A4 can be and how it's the little things that make it so much better to drive. It would be really hard to sell it. An Infiniti G35 would come close but they lack so much refinement.