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.
The most likely part that is causing the problem is the Idle Air Control Valve, IACV. It controls the idle at low speeds and idle as the engine load increases.
When you replace this part, check for vacuum leaks as well.