catalytic converter

I have a 1993 G Am and have just had the catalytic converter and muffler replaced, the mechanics put her on an analysis machine and nothing showed up as abnormal however it seems to be struggling with maintaining power and a warning light comes on once she is warmed up. I don't have the owners manual, it is the top left light under the temp gauge. Any ideas???

What does the light look like? A better discription would help.