False P0420 Reading?

[catwomanx]

Is it possible that changing the cat in my 2003 1.8T (65,500 mi) Jetta would mess up my ECM and require a flash? I've been getting a P0420 code off and on ever since that change out (not OEM part). Since there was an ECM recall I got the bright idea that maybe there was a possibility that the code is a misread since I have a practically new part. Seems too coincidental that the O2 sensors are bad when there was no problem before? Long story below if you care, but I'm trying to determine how to tackle this with the dealer without having a zillion things done unnecessarily!

In 2010 my flex hose from the engine to the cat broke and I took the (bad) advice to replace the whole cat (at Monro). I called the dealer and they quoted a huge price and told me they'd check the warranty. Called back and said the warranty was up at 5/50,000 - I was at about 52,000 mi at the time so figured there was no point in paying a ton extra if I had to pay for it myself anyhow. Got the work done and maybe a month later engine light came on. Went to dealer who got P0420 code and said non-factory cat - you have to get a factory cat. Brought it back to Monro and replaced it (non-factory of course). Seemed fine for a month or so and like groundhog day, check engine light back on...they ultimately ended up replacing the cat AGAIN but the third time was not a charm. In the mean time I found out there's a federal emmissions warranty for 7/70,000 and that the dealer was wrong about the cat warranty and it's actually 8/80,000. Argh. Last time I went to Monro they told me to go to the dealer for diagnostic...they're getting P0420 but allegedly can't see more than that? I don't want to pay the dealer to tell me it's a P0420 code...or just blow me off and say I need THEM to put in the new cat. Plus of course I'm pissed they told me it wasn't under warranty when it was. Grr.

[Twopnt016v]

Basically the cat they replaced the oem cat with was most likely a highflow cat or is not as efficient as the oem unit which is resulting in a cat effeciency code. Some software updates would correct the code you mention if a bunch of cars were throwing the code erroneously and vw decided to make a correction via the software. P0420 codes are the hardest to work thru since anything that makes the car run worse can throw a p0420 code. Anything that makes the cars emissions harsher could trigger the code...think about it...if its harsher going in the cat it will be harsher coming out. I've seen people replace the eom cat with another oem cat and still have the code. The easiest fix here is go to 42 draft designs and get an o2 sensor spacer. You install it on the rear o2 and it spaces it out so the gases aren't as potent going across the sensor and the car doesn't throw the code anymore.

the spacers are $30...remove your rear o2 sensor, thread the spacer in and then thread the o2 into the spacer...no welding required..

[Slimjimmn]

Please let me know the brand of downpipe you installed.

I have never had good luck with aftermarket pipes, they seem to only last 2-3years if that.