A scanner won't "tell you", it may give you hints.
For example..Maymybonneliveforever had a bad pcm. It allowed the car to run and drive. However it never showed the car as up to temp and was bascially dumping fuel down the intake. It ran poorly and acted crazy.
Changing the pcm was a "try" at seeing what was happening and it magically worked great the minute the pcm was swapped. It'* tough to determine if the pcm is bad w/o ruling out almost everything else..... which is about where you are.
PCM is under the passenger side kick panel (under dash), a big silver box laying in a bracket on the firewall.
00 Regal GS: All kinds of cool stuff.