2001 Audi A6 Cat code

This code hardly ever means your cat is bad.

Your car uses 4 oxygen sensors, 2 on each bank.

On one bank, there's one sensor before that cat and one after.

The computer uses the "Before" sensor to adjust (fine tune) fuel flow to the injectors. The second sensor is used to measure the converter's effectiveness at removing the harmfull gases out of the exhaust.

But when the one/both of these sensors begin aging, they give different readings that are out of spec. This sometimes fools the computer into thinking the cat is bad.

To verify your cats effeciency, you'll need to run an exhaust gas analysis. For the O2 sensors, you can check them with a DVOM. But if they haven't been changed through the life of the car, then you could use new ones anyway. (Old O2 sensors can also reduce fuel economy)