The onboard diagnostics system (OBD-I) is used to diagnose engine problems and other emissions-related issues with cars produced up to the year 1996, when onboard diagnostics were upgraded to a newer version called OBD-II. Checking for the OBD-I plug, or diagnostic link connector, in your 240SX allows you to plug in an OBD device which scans the system for error codes, each of which represent a unique point of failure within the car.
Turn off your 240SX's engine and sit in the driver's seat.2
Press your hand against the dashboard material, below the steering wheel on your left-hand side and feel down until you locate a small rectangular port that is approximately 2 1.2 inches wide and about 1 inch in height.3
Plug the OBD code reading device into the port and then turn your 240SX's electrical system or engine back on. Allow the code reader to automatically scan for error codes to complete checking for any issues.