Hyundai Repair: Check Engine Light on 2000 Hyundai Sonata, hyundai sonata, o2 sensor

Question
Thanks--where is the best place to have a trouble code checked?  Can I go to someplace like an Autozone or do I have to go to a mechanic?
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Followup To

Question -
I have owned my 2000 Sonata since 2002.  In the past 4 years, the check engine light has come on three different times.  The first time, I was told it was a bad O2 sensor which was a recalled item so it was fixed by the dealership.

The second time was last year and I was told I needed all new wiring and spark plugs.  After that repair, the light went off and the hard idiling I was experiencing went away as well.

The third time was recently and I've noticed the light comes on but then will go off.  While my husband was driving it over the weekend, the light came back on, we heard a slight thumping noise and it lost RPM's.  He said he felt as if he had to lay on the gas to make it go.  That was 3 days ago and it has been fine since and the light also went off again.

what could possibly be causing this?

Answer -
The things that could make your check engine lamp come on and cause the vehicle to not run correctly are too many to list.  The first step to finding the answer is to have the engine (and in this case, transmission also) trouble codes checked.  Once you know the trouble codes (and what they mean), I can tell you what sorts of things could cause that trouble code as well as the symptoms you describe.

Answer
AutoZone has advertised that they'll check codes for free, but I've seen several questions here that lead me to believe that they cannot read the transmission codes.  Since it's free, you can go there first and see what the engine code is.  If you have a P1529, that indicates there's a transmission code present and you'll need to go somewhere that can read transmission codes.  Since I work at a dealer, I'm not familiar with the generic scan tools and don't know how readily they can read transmission codes.  You should probably call prior to taking your car to a shop other than the dealer to see if they'll be able to read your transmissin codes (if you have a P1529).