as Mountain air said put a code tester on it.
You can get a OBD-II to blue tooth adapter.. I've had about 50/50 luck with em. Hold on...
EbayJust plugged it into my Jeep.. Fired up TORQUE on my phone (They also have a diagnostic program but I already have Torque) and it worked.
HOWEVER.. i had one incident where Torque messed up the car's computer. it's not supposed to do that but it did. Only happened once and I was configuring the display so I may have picked an option that the jeep did not like.
Never had an issue with my older rides though