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See picture below of the location of the port:
Try the link below:
That code is for cat efficency which means that the cat isnt removing enough pollutants. The only thing that can be done about that code is first to have VW update the engine computers software. If it still comes up afterwards, then the cat is bad. No valves or vacuum lines the problem pertaining to that code.
All sensors are ok. The software update is about an hour of labor.
Go with the Mobil 1 Euro Car formula. I use it in all of my vlients VW's and have had nothing but positive results under the most extreme conditions and it is highly resistant to sludging. As for the octane, use 89 or above at all times. Also, if the light went out, then you are fine and good to go for now.
Mobil 1 only comes in 0W40 now so its pretty easy to distinguish. You can switch back and fourth as much as you like, but M1 is way better for your engine and you should stick with it. If you choose to use conventional oil, 15W-40 is the only weight you should use and not a diesel formula either.