A simple problem, really... Oxygen sensor going bad?

Yes. Did he make trouble-

No.

Your car should have two sensors. How to diagnose and repair depends on the mechanic and your decision. He can spend an hour or more at $100 per hour diagnosing or 20 minutes replacing both of them. Diagnosis includes reading the O2 voltage at various temperatures either at the sensors or at the OBD port. He can also remove them and use a torch on them while measuring voltage. However, if it were my car with 102k miles, I would replace the O2 sensors. They tend to get polluted and stop working right about now.

What was the number?

If this is what I suspect it is, it means that the downstream oxygen sensor _after_ the catalytic converter is giving a high reading.

This could be because the sensor is bad. It could be because the converter is bad. It could be because the engine is running rich but the upstream sensor has yet to go out of range.

He made it harder IF you don't know what the error code was.

A good mechanic will put a scanner onto the port and actually look at the values coming back from the sensors, and by looking at both the upstream and downstream oxygen sensor values he can make a good guess about what is going on. He might also stick a sensor up the tailpipe and get an accurate reference value that he can compare with what he sees from the downstream sensor.

A bad mechanic will just swap things out until the problem goes away, starting with the oxygen sensor, and then the converter, and then something else random.

Your car is at about the age where the converter is a likely suspect.

--scott