it's nothing, but because you disconnected something while the car was on, the car automatically turns on the check engine light because it believes there's something wrong.. I don't thing there's a problem...try resetting your ECU and if it comes on again, thn theres something wrong.......unplug the negative battery terminal for about 10 mins,..plug it back up and start the car,..DO NOT touch the gas pedal,..let the car idle for 10 mins,..shut her off and then your ECU will be reset