How do you test diagnostic codes on a Dodge Chrysler or Jeep vehicle?
Here is a great place to Do simple diagnostic test with your key and dash lights / odometer. http://www.allpar.com/fix/codes.html
Diagnostic codes are generated by your on-board computer. They tell you what problems the computer has identified with your engine. The codes are numbers. The code numbers are read by an instrument that plugs into a special jack under the dash. Auto parts stores often will check your diagnostic codes for free, or you can have your dealer do it.