I recently changed my alternator and now the warning light on traction control on 2004 Pontiac Grand Prix GTP

is on WHY ?! can it be read by a computer ?

Short anwser is it may have nothing to do with alternator...but codes can be read with a few scanners but not all.
1 more answer
you may need to reset the computer...all you have to do to re-set is remove the neg cable from your battery for 10 min. and then reconnect. hope this helps