I'll give you the whole sequence of events... cel comes on I get codes from autozone 2 cylinders misfiring. Apparently there was oil in the spark plug wells.. I replaced the valve cover gasket, replaced the plugs as well as coils.

2 weeks later cel is back! Code p0420 I do some research & see that it may be the cat especially since the oil was in there I thought that was it. I replace the catalytic converter cleared cel. It comes back a week later.

I was about to change the o2 sensors but figured id take it to the shop to see if I really need to. They say the o2 sensors aren't the problem & I had to loose vacuum hoses & a couple of dirty grounds. They charged $135 for the fix. Cel stayed off for 110 miles & now it's back. What else should I check!?

 

Even though nobody even remotely tried to help here's an update just incase someone else ever has the same issue....

Usually if u buy an after market catalytic converter I guess the specs aren't perfect for the audi's needs. You can get an adapter for $69.99 that will make it work properly & clear the p0420 code & thus allowing you to pass emissions (if your state tests that).

I don't know where to buy the part I took it to an Audi/VW specialist & they solved my problem