The code p0421 (warm up catalyst) came up on my 2001 A6. What exactly does this code mean? I deleted the code and so far it has not come back on the same day after driving it.

 

It may be a sign of CAT converters going bad, or Post CAT O2 sensors going bad. Try to replace Post CAT O2 sensors first. It worked on my car. You can also try a trick with a diode to save money on new sensors. The trick works fine. I tested it. Unless there's something else going on with sensors, you should not see these codes anymore.

 

Thanks a lot if the code comes up again I will try replacing the sensors first.