2005 Passat-CEL & P0431 Codes

I could use the assistance of the hive mind here with some codes after a CEL popped up. I had posted a question back in late February/early March with a CEL that turned out to be a flex pipe issue. Local muffler shop fixed the pipes, the CEL disappeared as did the VROOM VROOM sound sound that came with acceleration. No performance issues with the car since then. The help from the group was incredibly informative both on the diagnostic side and with price quotes from shops.

Two weeks ago, the CEL came on again. No performance issues or noises coming from the car this time around. I did notice that after parking and killing engine, there is a faint exhaust-type smell though. I took the car to my local Advance Auto Parts and had them hook the car up to their computer for a diagnostic. Two codes came up.

#1: P0431, warm-up catalyst efficiency bank 2 below threshold pending
#2: P0431, warm-up catalyst efficiency bank 2 below threshold confirmed

The AAP employee didn’t have anything to add to the code readouts. I’m concerned that this is a catalytic converter issue, and if that’s the case, a repair would likely cost more than I’m willing to put into the car at this point. It has just over 125K miles on it and not due for emissions inspections until October. I have not taken it into a shop yet, hoping to get some knowledge here before doing so. Any insight into the codes? And what action to take with bringing to a shop in terms of repairs and estimates? As always, all thoughts and comments welcome. If more information is needed on the car itself, please let me know.



Time for dealer intervention. I responded to a post not too long ago, left bank lean, took it to my guys and they said take it to the Ford dealer, Ended up the dealer had better code reading info and had to replace plenum o rings. Now I know a lot about old cars but would have no clue where to look for a plenum o ring, or even diagnose it is bad.

Your car has two oxygen sensors: one upstream of the converter, and another downstream. This code is saying that the downstream O2 sensor’s reading are not within spec.

There’s really only three possible causes I can think of:

  1. There’s an exhaust leak, which is throwing off the readings
  2. The O2 sensor is question is faulty
  3. The converter itself is faulty

You said you smelled exhaust, so I would start with looking for a leak somewhere. If there isn’t one then the sensor should be tested for proper operating voltages. That will tell if the sensor or converter needs to be replaced.

EDIT: Since the code mentions Bank 2, I assume you have a V6 engine? In that case you must actually have two converters and four O2 sensors. Bank 2 will be the side of the engine that does not have the #1 cylinder.