An OBD II scanner hooks up to your truck's ECU and allows you to query the processor for information. If you click on the link in my post in Parts and Accessories it will take you to the site where I bought mine. There are pictures and specifications available there. You can get trouble codes and reset them as well as monitor a lot of stuff going on with your truck. There are many other units available elsewhere. I'm having fun with mine and it will pay for itself the first time I diagnose a problem and reset the code myself.