There are two types of diagnostic problems that could afflict a Jeep Liberty. The Liberty's internal computer could be working fine, and the diagnostic problems may be the trouble codes logged into the system. The Liberty's powertrain control module routinely checks the engine, electrical, and other systems and catalogs malfunctions. The other type of diagnostic could be the PCM itself. If it is not working, then the Liberty's check engine light will no longer function, and you will not receive advanced warning of problems prior to vehicle breakdown. The process of checking for both is the same.
Attach your OBD-II scanner to its diagnostic cable. Then, attach the cable to the Jeep Liberty's diagnostic computer outlet. This data link connection will be found beneath the Liberty's dashboard, directly beneath the dashboard.2
Switch the electrical system on. Place your key into the Liberty's ignition and turn to "On." You may also be required to crank the Jeep's engine, depending on the brand and model of handheld OBD-II device you won.3
Wait for your scanner and the Jeep's PCM to interface. This should take only a second. However, if you own a lower-end OBD-II code reader, you may need to tell you device to retrieve the codes. The exact process for entering commands will be in your device's manual. The steps on how to enter these commands also differ by device-brand.4
Copy out all the codes retrieved by your diagnostic hardware. You do not need to copy any of the codes marked as "pending." These seldom occurring problems are not responsible for the check engine light activating. However, if you want a complete history of faults that have happened, you shoud investigate these codes too.5
Switch or scanner off, and remove your OBD-II device's cable from the Liberty's computer outlet. Shut down all running components within the Jeep and exit the vehicle.6
Research the codes on your list. Due to Environmental Protection Agency mandates, all OBD-II compliant vehicles use the same generic codes, which are usually listed in your scanner's manual. Jeep, and all other Chrysler vehicles, also employ a supplemental group of diagnostic codes. Neither your scanner's manual nor your Jeep's handbook will have these. They can be found online or in specialized, make, model, and year specific repair manuals. If you are using a Haynes or Chilton book, most of the OBD-II codes can be found in the sections dealing with vehicle emissions.7
Match codes from your Jeep Liberty's PCM with definitions you have found in Step 6. If you are looking for general diagnostic problems, the lists you researched should suffice. If you are looking for issues with the On-Board Diagnostic system as a whole, read through definitions and find codes that mention the PCM. You will such codes not only listed under "Powertrain," but under "Chassis" as well.8
Consider each OBD-II code definition currently active in your Jeep. If you have the knowledge and experience, you can do some of the repairs yourself. If you have found codes indicating a faulty PCM, you may need to reprogram the module. You may also need to remove and replace it, if it is beyond repair.