2004 Jetta GLS throwing codes P0102 and P0420
 

Check engine light came on yesterday on my 2004 Jetta GLS; originally thought it might be because my daughter put Reg. unleaded in it and did not tighten the gas cap but took it to Autozone today and it threw codes P0102 and P0420..........the Autozone guy said that there is a good possibility that both codes are being thrown for one reason (maybe a loose line?)
Any help appreciated!

Did the guy even explained to you, what the codes mean??
P0102 means: Mass or Air flow circuit low input. The solution might be that there is a leak somewhere in the air intake. Make sure to check the air intake booth for any holes.
P0420 means: Catalyst system efficiency below threshold Bank 1. It could be the O2 sensor that quit working.
But try to find out first if you have a leak in the air intake.
If you need any more advice, just let me know. :)

Ok, first off, using AutoZone to get the codes read for free is a good idea. However, listening to anything that the guy at AutoZone says about the codes is usually a bad idea. They are geared with the thinking that every code is a part that can be sold. In your case, with a Mass Air Flow sensor code, you will want o ignore the Catalyst Efficiency Code until you deal with the MAF sensor. Usually, the low or high input codes are caused by a bad sensor, however a wiring diagram, multimeter, and some time testing the sensor would be a good idea before replacing. After you replace the sensor, clear the codes and let the computer run it's Catalyst monitor again (this could take a while, maybe a week or 2) and then see if it turns the light back on.

get a can of mass air flow sensor cleaner for about $6 from autozone, napa, oreilly, action autoparts, or any other auto parts store. follow directions on can to clean your MAF sensor.

my car (2003 toyota matrix xrs) threw the p0420 while still under warranty. i took it to dealer and it was a bad catalytic converter. it doesn't hurt the car to drive it with a bad cat, but you will have poor fuel economy for it. cat. converter was replaced on my fiance's car (2000 jetta 2.0), and it cost about $250 out the door as i recall.

It is never a good idea to run the car with a bad cat for a few reasons other than the pollution. Yes, the car will default to rich condition and consume more fuel. Your check engine light will also stay on. If something else goes wrong, how would you know? This could cause you to harm your engine, eg., if your thermostat or coolant temperature sensor starts going bad, you wouldn't know because you would be used to the light always being on.

Also, replacing the cat with a cheap "oem" replacement or generic from the local autoparts store will not cut it. By law they do not have to function as efficiently as the factory cat and your check engine light will will eventually come back on and put you right back where you started.

I just replaced the one on my girlfriend's '03 jetta gls 2.0L and it was well over a $grand$ after diagnostics and labor.

i agree, it isn't a good idea to run with a CEL. in this case, the pollution increase will be minimal, and you won't have any real harm come from driving around with bad cat for a few days to a month to get money together for repairs. OEM doesn't mean cheap necessarily. OEM is Original Equipment Manufacturer, meaning the stock part like what came in the vehicle when it was first built, AKA stock parts. aftermarket can be just as good while being cheaper than OEM, because it isn't always the big Brand Name. also, a less efficient catalytic converter may give the CEL again in the future, but chances are that you will have gotten rid of the car by then. if it is already a 10 year old car, and you replace the Cat now, do you think you will keep the car for another 10 years with the same amount of driving? probably not.