Try the key method. I had to do this when my scanner pulled no codes with the check engine light on. You turn the key to ON but don't start the Jeep and then turn it off a few times. What you do is insert key,(1) turn to on, (2)turn off, (3)turn on,(4)turn off, (5)turn on and leave it. Your odometer will then display a code(s) and then say done. If it goes straight to done there aren't any codes. If you accidentally turn the engine over you have to start over.