Advance Auto and Auto Zone do scan codes for free however that doesn't go far enough to buy your own parts and fix the problem. The sensors and wiring in the suspect system needs to be checked out using a volt meter or mechanic's scan tool. Remember that the person that reads the codes is a sales person and not a mechanic!!

Many parts stores like Autozone offer to scan for trouble codes for free but only on 95-96 and later model cars/trucks with the OBD-II systems. Call them to verify if in doubt. They can usually also clear the codes if you need to see if they come back or after doing repairs and can tell you what various codes mean or what to look for in doing repairs.

Sites like OBD-codes,com also have info for various codes and car models.

http://www.obd-codes.com/

Depends where

like where will it be free??