1999 with only 157 miles. Cool. (just funnin')
The code means "catalyst system efficiency below threshold".
Normally it would mean either the catalyst has been compromised due to exhaust stream deposits onto the platinum palldium over time (cat connverter shot) or the downstream oxygen sensor(s) is defective.
Which O2 sensors were changed? Both upstream and downstream?
Why was the work done?
And, since you had it done rather than doing it yourself, have you brought it back to the shop to look at?
Normally what I'd want to do is look at the signals from the oxygen sensors themselves.