2004 Pontiac Grand Prix

Engine Performance problem
2004 Pontiac Grand Prix 6 cyl Front Wheel Drive Automatic 50000 miles

I just got my car back from Aamco and had the transmission replaced, both front wheel bearings replaced, thermostat replaced. Well, 4 days and $3800.00 later my display inside the car says REDUCED ENGINE POWER. Before I pulled out I asked the guy why this is on and what's wrong. He just reset the display and said Oh, that's nothing. I am a little confused--They kept the car an extra couple of hours because they could'nt match up the sensors.
I have a feeling they broke something___what do you think it is? Can I drive the car like this? And how much more money do I have to spend to get this fixed? Should I have Aamco fix it?
February 28, 2010.

Yes have Aamco, fix, reduced engine power indicates a problem with traction control system.

Mar 4, 2010.