JohnKo

MIL on. Just one DTC -- P0421. Alleged to have several possible root causes. A search hasn't turned up too much. Any experience out there? Mileage is high enough that I can't rule anything out on that basis. Hate to just start throwing expensive parts at it.
TIA

Arcteryx::Stock Rods

it's your cats. if you do a search for "P0421", you'll find your answers.

AVANT S4 KING

What how many miles are on the car?
Are you chipped?
Is the car still using the original O2 sensors?

My fiance's Jetta threw P0420 and after a bit of research and found that her cat was covered until 100K miles (thanks to a design flaw and some strong-arming by the EPA, it was extended from 70/7 to 100/10 on that model). I took it to the dealer and they had it back to us the next day.

If I had to guess, I would say that you just need to replace your front O2's, especially if you are chipped and still on the original O2's.

HIS4

At first, it would take about 3-4 weeks for the code to come back after I cleared it but now it comes back in a few days. My primary O2 sensors are new but the secondary sensors are still original with 118k miles on them. I'm still debating whether I should attempt to replace the secondary O2s first or just get new DPs and cats.

JohnKo

Thanks, guys.

Not chipped, but 120k on OE cats and sensors.

Using the search function, I had only turned up three threads, which is why I asked in the first place. Tried again and got many more -- not sure how/why. I've also learned that the generic stuff is vague, but the factory information points directly to the front cat.

HIS4, my MIL behavior was similar. Keep in mind that this DTC refers only to Bank 1.