OK with the 0421 - I am assuming a bad cat. So I guess I could:
- ignor it
- try some catalytic cleaner
- install a used cat
- try the VAST piggy pipes.

Car (2000 A6 2.7T, 6-speed) has 116K. Some blue smoke on start-up, burns a little oil, and mileage has slipped a little; from 20-21 to 17/18 in the city.

 

Good luck in finding a used cat b/c I think Fed forbids the used cat sale and a $100k fine.

You may check this out www.bigdaddiesgarage.com

 

I think P0421 and P0431 codes are set to come on too soon on our cars. If your mileage is not highly impacted, and you know that the converters are not really plugged, the you may be able to mask the issue by using spark plug anti fouler trick. Otherwise break open your piggie bank.

 

Often these codes come from bad O2 sensors. You need to get the car on a real time scanner to monitor the sensors with the car running to see how they are working. They are cheaper than Cats.

 

Also, the codes come from bad gas, particularly as the cold weather sets in and causes excess condensation in the underground tanks. My codes went away after a couple of tankfuls of cleaner gas.

Bob