2003 Toyotas use a system known as on-board diagnostics, or OBDII, to ensure that the emissions equipment is functioning properly. The engine computer is constantly gathering data whenever the vehicle is running. If the computer senses one of the components of the emissions systems is malfunctioning, it will illuminate the check engine light in the dash. The check engine light must be turned off after the appropriate repair has been made.
Park the vehicle on a level surface and set the parking brake.
Disconnect the ground (black) cable from the negative battery terminal by loosening the retaining bolt and sliding the ground cable off the negative battery post.