What auto parts stores do free diagnostic testing?

O'Reilly Auto Parts, Auto Zone and Pep Boys are auto parts stores that offer free diagnostic testing on check engine lights. These three national chains have retail locations all over the country.

During a check engine light diagnostic test, a store employee scans a vehicle's on-board diagnostics system with a handheld device that brings up any diagnostic fault codes that are stored in the computer's memory. These codes help narrow the list of potential causes and make troubleshooting easier. If the issue is minor, visiting a local O'Reilly Auto Parts, Auto Zone or Pep Boys store can result in a less costly fix than going to the mechanic.