What causes low idle at stops (e.G. Red lights)

My 2001 olds alero idles low (approx 300-500)and almost dies at red lights. A pepboys center said the car generates no codes when tested on the pepboys computer. They do not know the reason it passes the diagnostic test, but still idles low. Also, when idling low the serpentine belt chirps, but they said the belt is working properly.

Please help diagnose the problem.

joseph