My friend just got a b6 and he has a CEL. These are the codes that he pulled with the obdII. I can see that he has 2 check valves that are broken and I think two of those codes are for that but one of them says that it has catalyst deficency. Could that be because of the check valves, or does he have a bad o2 sensor???

 

P0420 is a bad Cat converter. Its fully covered under warranty for 8years/80k miles. I just had mine replaced last month 100% covered, I was @ 77k miles. Its not cheap if he has to pay out of pocket. Probably looking at $1,500 + for the job @ the dealer with an OEM Cat.