Eric bach

We recently picked up a used GMC Sierra 1500 from a reputable dealer. Yesterday after a fishing trip i came home and put the truck in park and it would not shut off with the Key out of the Ignition and the ignition in the off position. I believe it to be an electrical issue because after we finally figured out how to get it to turn off I removed the battery cables and noticed the check engine light turned off with the ignition in off and when i reconnect them it comes back on in the off position. Any ideas on where to start?