Hello All-

I purchased a 2001 A6 Avant with 120K miles last March. Two weeks ago, CEL came on and took it in to a European shop in town. The initial diagnostics were P0441 ( EVAP- incorrect flow) and P0420 (Catalyst System; Bank 1: Efficiency Below Threshold.
The Fuel Evap was replaced and CEL went away. After driving it for a week, the P0420 CEL came back on. Any Idea what P0420 is???

thanks in advance.

 

one of two things.. either the post cat oxygen sensor for bank number one is degraded and not measuring properly or your bank one catalyst is no longer reducing emissions as it should.(the latter is usually the case)

 

bigern45-

I looked underneath the car...and I only saw one O2 sensor each of the Cat...one sensor is somewhat mounted sideways on the Cat (driver side)... and the other one is mounted on the top of the Cat (passenger side)

Which one is bank 1?

thanks for your insights...

 

im assuming you have a 2.8??? or is the avant a 2.7t?? anyhow.. bank number one is on the passenger side...

 

It's a 2.8 avant. Would you happen to know if there's an DIY article on how to remove the cat from the header back?

thank you in advance.

 

im sure there is .. but im not very good at finding those things.. sorry.. im sure you can use the search as well or better than i so you should be able to find it..

 

I found few pictures from Autozone's website. It shows where the O2 sensors are located.

One O2 sensor (passenger side) is located near the head gasket area mounted right after the the exhaust manifold under the hood. The other O2 sensor (passenger side) is underneath the car right before the cat. Please correct me if I'm wrong.

I might tackle replacing the the O2 sensor first than the cat...don't know which one yet...but, it's the cheapiest route...

thanks