On my A6 Avant, I have been getting these same codes since at least 80K in 2004. P0421 (Pri Cat Bank 1), P0431 (Pri Cat Bank 2), and P1423 (secondary air flow too low bank 1) always show up 1 or 2 days after being cleared.
I can guarandamtee you these cats did not go bad at 80K.
How to know? I always knew those emissions tests in Denver would be good for something. In 2004 at 66K, my hydrocarbons were 0.07 gm/mi and NOx was 0.2 gm/mi. In 2006, at 136K HC were 0.13 gm/mi an NOx 0.3 gm/mi. Pass/fail cutpoints are 1.2 for HC and 3.0 for NOx. This car is pretty darn clean from an emissions standpoint.
If the cat was shot, HC would be much higher, trust me, people I know who have had bad cats have failed the emissions test hands down. Even if my cat is bad, why would I replace it if my car runs that clean? That makes no sense.
I need another emissions test this year (177K) so will do that before taking to my mechanic this week. I will try to have him do a pre- post- cat temperature reading.
The fuel economy on this car has always ranged from 20-22 mpg, so I don't believe it is an O2 sensor. I believe it is a secondary air leak issue that is of very minor concern (except for that damn CEL/MIL). Hope to have a diagnostic on that this week.
Will let you know what transpires.
Sunday, February 17th, 2008 AT 10:02 PM