Toyota Repair: Check Engine Light on 1997 Camry, trouble code, o2 sensor

Question
Hi, I have a 97 Camry, with almost 100K, and a whole 128 point service inspection was done about three months ago and since then I put about 4000K on the car. Just the yesterday the check engine light came on. And me not knowing too much about cars didn't want to take any chances so I went straight to a mechanic, who also performed my state inspection last week. After doing a diagnostic they said it was the 02 sensory, so I say go ahead and replace it. After they were done the check engine light wasn't on anymore so I was all happy, and drove the car straight home, about a quarter mile from the mechanic. Then this morning I turn the car on for the first time since I got back from the mechanic and the light turned on again. So besides going back to the mechanic and demanding that something should be done, is there anything else I should do?
Thanks...  

Answer
I need to know two things:
Is it a 4cylinder or V-6 engine and what was the diagnostic trouble code(DTC)that was in the computer that made the mechanic replace the O2 sensor, without this information I can't help.
I recommend that you go back there and have them retreive the trouble code and then let me know what it is, since they obviously didn't repair the problem they should, in my opinion, do this at no charge, all I need to know is the code and I will advise from there.