2005 Saturn Vue traction Light

Hi, I am hoping you can help me. I have a 2005 Saturn Vue with about 9250 miles on it. My problem started after having the car for about 6 months. It first started with what sounded like the Engine running when I was sitting still or in Park. I took it to Saturn to check, and they could not find the problem. About 6 months later, my Traction Light would come on when I was in Park. I once again took it to Saturn and at first they could not find anything wrong with the car. Saturn ran several tests and could not fnd anything wrong with the car. FINALLY they figured out that the Traction Light was coming on due to a bad ABS/Traction unit, in wich Saturn replaced for me. Promising that the light will not come on again. Well it started again about 1-2 weeks ago. I was in Park when the Traction Light would come on, however the light does not come on all the time. and now when the light comes on I can not push the traction light button to reset it. Nothing happens. So, once again I take the car back to Saturn, however this time they say there is nothing wrong with the car. My question to you is
Can you please let me know why this light is coming on, and what does it mean ??
Why when my car is contacted to the Diagnostic Machine it does not detect anything?