99 GP SES code

I just bought a used 99 Grand Prix SE. 2nd day I had it the Service Engine Soon light came on. The used car dealer I bought it from told me that if I
ever had an ses code to bring the car back and they would check the code at no cost to me, but they don't have a shop to work on things. Kind of a "service" for those who like to do repairs themselves. Problem is they didn't recognize the code and the guy cleared it before I could write it down. It said something about catalyst low or something like that (sorry for the lack of detail).
As near as I can tell, this is going to be an o2 sensor issue. There are only two on this car, correct? Any way to tell in advance which of the two it is?
Since buying the car, I've learned that the sensors in these cars can be a bit of a problem. At least in my opinion if night tightening a gas cap sets off the sensor, that is too sensative of a system. I understand that grade of gas and gas quality can also be an issue. I did buy gas immediately after I bought the car from a Phil 66 station. I don't remember the octane level, but it was the cheapest of the three pumps. I think I put in about 13 or 14 gallons of gas. Could this have set off a code that has something to do with the catalyst? I've read that it will take at least one tank of "good" gas to reset whatever code comes up from "bad" gas.
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If they pulled a code P0420 (Catalyst System Low Efficiency) out, and no others, then in all likelihood it is the catalytic converter that's the cause of the problem. With the way GM runs the diagnostic for that code, it's unlikely that a bad O2 sensor would cause it (a slow-responding upstream O2 sensor would set a code for itself, and a slow-responding downstream O2 sensor would cause the converter efficiency to look better than it was, not worse).
It'd be unusual for a '99 to have a shot catalyst unless it has really high miles, or unless maybe the previous owner drove around with the engine misfiring for a long time and burned it up..
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