mwspera

We replaced both the front WRAF sensor and the back O2 sensor AND the CAT (with a new high quality replacement). Cooling system was pressure tested and found to be absolutely tight (no pressure drop in 2 days). Car burns the usual amount of oil for a 1.8T. No smoke out of the exhaust (blue or white). Fuel economy is at 29.3 overall (O.K., I drive like a granny). 4 months after CAT replacement, the P0420 is back. Nothing unusual with the Vag-com hooked up.

What now?
Thanks,
Mike

donnyt

I had the same problem when I installed a HFC, the solution was easy - I just needed test pipe software on my ecu from APR to bypass that same code.

initialA4

The "high quality replacement" cat is what caused that code to trigger. Usually that code means that the cat is bad.

mwspera

Um, well, yes, that is what the code usually means. My question is, why did I have that code with the original, and then replacing the cat and all the other "usual suspects" yields the same code again after 4 months? It is possible that the original was bad AND the replacement has also gone bad, but the usual causes for that (bad sensors, lean running, coolant leak, high oil burning, exhaust leaks, melted/broken cat media) are not present. I think I have established that the ECU believes the cat is bad. I am trying to determine why both may have GONE bad (if they really did). The original lasted 110,000 miles, the replacement lasted about 4,000 miles. This car has absolutely NO mods and has been driven VERY gently over its life. The "cat sites" all tell of cats initially losing about 3-5% of their efficiency after a few months. That may explain why it worked for a while. I now must determine why it has crossed the threshold BEFORE we invest in another cat. If the replacement was defective, so be it. But, at these prices, I cannot afford to keep replacing them as a "test". I would pay for an OEM unit IF I was convinced that this would cure the problem. If something is causing them to go bad, I would like to cure that first.

Any clues?
Thanks,
Mike

initialA4

I believe that your replacement cat has the wrong load, causing the CEL. Pretty much you bought the wrong cat though.

mwspera

Wrong cat? It is a direct fit replacement, not some cobbled together universal. You would never get a uni to fit in there.

vismaf67

Did you ever got the P0420 code to go away?

infamous107

Well after months of battling this “p0420” code myself, I FINALLY found a solution to get rid of the check engine light after replacing part after part associated with this code. I was about to give up and hand over 1400.00+ to the Audi dealership until I ran across one more solution. I started reading on information concerning an O2 spacer for the second bank. This spacer I guess, gives the O2 sensor a gap so that the temperature coming out of the aftermarket catalytic converter reads within limits and “poof” engine light be gone!!! So once again I have to thank this site on saving me about $700.00 and some change. Took me a little to figure this one out, but the cat was initially on the quits. So if you guys are on the same troubleshooting boat with this code and you have done EVERYTHING in its power, give the O2 spacer a try. I bought it for 23 bucks off a german parts website. Now awaiting my next check engine light troubleshooting issue…lol.