2012 GTI with 102,000 miles. The check engine came on recently, and after borrowing a code scanner from a coworker got the P013A code. So my questions are is it better to take it to a dealer or is it something that I can easily handle myself, since I don’t know where exactly they’re located. I know in some cases these things are underneath something else that has to be removed. If it’s better to take to a dealer, how much does that run? If I do it myself, what’s the best brand? Thanks.
That’s the down-stream O2 sensor. Or the one after the catalytic converter,
This means the vehicle has to be lifted in order to replace the O2 sensor
Tester
Suggest to have a shop do a proper diagnosis. They’ll examine the O2 sensor signals from the upstream and downstream sensor with a professional scan tool.The problem could be the sensor, but it could also be the wiring to the sensor, an exhaust leak, or the cat needs to be replaced. Have you been having any misfire problems? Any other engine or drive-ability problems?
If you decide to replace the sensor, google to see what brand VW uses for the oem part. Probably Bosch or Denso.
German car… VERY nationalistic manufacturers, isn’t likely Denso! 
But I agree with George, cold be a number of things. If you just swap out the sensor and it doesn’t fix the problem, you’ll be posting here again wondering why that didn’t work!
OK. Two follow ups, how much does it cost to have it fixed at a shop? I’m not arguing against doing it at a shop, I just need to know how much to expect.
Also, the light turned off yesterday when I was out, why is that? The code is still registering.
I partially disagree with that
When I was working at the Benz dealership, most of the cars were equipped with Denso AC compressors. The ML-class was built in USA, and quite a few of the electronic modules weren’t German. And let’s not forget the MIchelin tires. Last time I checked, Michelin was NOT a German company . . .
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I’d guess the Denso compressors were built in the US near the ML plant so in they went. Even the Germans can’t ignore that efficiency! 
Same for the Michelin tires. US built and delivered.
I know Delphi sold a number of ML parts as well. Lots of local content that MB wouldn’t use in their cars but would in that truck made in Alabama!
I was told the car guys looked down on the SUV designers. Don’t know if that is why they used those sources… 
The Denso compressors were used across the board . . . even on the models built in Germany
And what about those Michelin tires . . .
If they’re so nationalistic, why not insist on German brand tires across the board
Mercedes isn’t even a German name
if they were truly as nationalistic as you suggest, surely they would have come up with a different name for their product . . . Why not Otto Schmidt, since that sounds more German, no ambiguity at all
one thing’s for sure . . . your “theories” are insane 
There is no way to guess at a cost. Any shop is going to their own diagnostic to make sure they fix what is really wrong. Those cost about 100.00 to 125.00 . And no you don’t need a dealer .
For the CCTA 4 banger 2L turbo, I’m seeing $200-$250 for each oem replacement sensor, and 1/2-1 h labor for each. If you want to diy, make sure the parts shop gives you the correct sensor. There’s 3 versions, D1, D2, and D4. Compare the new one to the old one. O2 sensors a quite robust, so it seem pretty unlikely this is a sensor problem on a 7 year old car. Unless there’s been some prior problem affecting the air fuel ratio or excessive oil consumption.
I’ll look for a shop close by me here. I have noticed the gas mileage going down recently and when I park and turn the engine off, I notice a slight fuel smell in the air.
But why would the light go out?