When something goes wrong in the car

Go to any auto parts store, smile at the dude behind the counter and ask him if he could replace your bulb for you. He will _gladly_ take it from there. (He would do it for dude too, but the pretty smile always helps).

:-)

You would need to have someone scan the body computer to check for trouble codes. Most auto parts stores can scan engine computers but not always the body computer.
The trouble codes direct you as to what to test.

There are several thngs that can cause this. Your best bet is to get scan your car with a VAG-COM and see what the trouble codes are. Most auto part stores (like autozone) will scan them for free, some smog check stations will charge you a small fee to do it, or, there is always the dealership. You can also check the VAG-COM locator to see if there is someone near you that will let you use theirs.
Vag Com Locator

the following links can assist you with causes and repair
P0420 Catalyst System Efficiency Below Threshold https://www.autocodes.com/p0420.html P0420 OBD II Trouble Code

Not sure of why you are asking the question, so I'll give you my two cents:
1. Most any place selling you a battery can tell you what size battery fits in your car. Sometimes, more than one size will fit.
2. In general, always put the biggest series of a battery that will fit. For the same manufacturer, same style/model of battery, bigger is better. The more lead, the more acid available, the stronger the battery, the longer lasting power to start your car.
3. How long are you keeping the car? If you plan to get rid of it in a year, then don't spend money on a 72 month battery. Get a cheap one, almost any battery will handle a year as long as the alternator & charging system is working right.
4. Can you change it yourself, or will you have to pay someone? If you can do it yourself, best deals are usually had at an auto parts store rather than a retail or auto repair shop. Walmart may have a better price than any, but less likely to find someone who can help you determine what size battery for your car.
5. If you have to pay someone to install, consider whether you travel a lot. If you may need to get the battery replaced while you are on the road, a large chain is more likely to be there to help you. (This holds true for a do-it-yourself battery replacement as well as if you get them to replace it. Autozone is pretty much nationwide I think, Joe's Bar, Grill & autoparts may not be. (((SMILE))))
6. Finally, most autoparts stores will test your charging system for free. I think a lot of mechanics may do the same IF you buy a battery from them. It's a good idea to have this done to make sure the problem is just a battery and not a faulty charging system.

Not sure if that helps, but hope somewhere in there I answered your question.