2003 Pontiac Vibe Check engine light

My check engine light came on and we checked it at home with a diagnostic tool. The code that we received was "catalyst system efficiency below threshold bank 1". I took it to a mechanic and they told me that I needed a new Power Control Module because there was no communication with their scanner. They also told me I need a new catalytic converter. The only problems that I have experienced with my car is that the check engine light is on. It seems to run just fine. I can't figure out how we were able to get a reading on our diagnostic tool at home and they were not able to at the dealership. If my power control module and catalytic converter were both bad, wouldn't I be experiencing some sort of car problems?