On my 03 Audi A4 1.8 t I just changed to a high flow cat. It was $431.00 and had it done at 034 Motorsport in Fremont Ca. Did they change the location from 03 to 04 because mine is located on the passenger side not the drivers side. It kind of sounds like to me that you have experienced the infamous bad coil pack syndrom. I am dropping mine off tomorrow to the dealership for the voluntary factory recall for coil packs. Christian at 034 Motorsport said it happens all the time and suggested going to a high performance coil pack set that is $140.00 after the ones replaced from the recall fail again.

 

This is great to know. My car just threw out P0421 and P0431 codes today. Wouldn't you know my car has 133,500 km's on it!

 

try seafoam instead can tire additive.