I had a tranny put in my 2000 sable and afterwards the check engine light was on and the sable would not idle, warm or cold. A mechanic friend of mine checked the vaccuum hoses and both of theL shaped hoses had huge holes in them. He fabricated some for me and put them in in under 20 min. the next day. The check engine light went off and she idles great now! wonderful mechanic but i'm in a different town now.
Check out the RepairPal article on the Check Engine Light: http://repairpal.com/check-engine-light
That'll get you started. It could be a number of things causing the problem, so you'll need to seek out a good diagnostic technician.