The "check engine" light on your VW Jetta comes on because there is regular maintenance required or there is a failure of one of your Jetta's critical engine, fuel or emissions components. The on-board diagnostic computer generates and stores an error code that triggers the "check engine" light to come on. To diagnose the problem, you need to be able to read the error code. To do this, you need an on-board diagnostic scan tool, or OBD tool, which is available at most auto parts stores.
Open the fuse panel cover on your Jetta by pulling the fuse panel cover off with your hands. You may have to give it a good pull because it is clipped to the dash.
Locate the open port for the OBD tool. There is only one port large enough to accommodate the OBD tool.
Plug the electrical connector from the tool into the fuse panel.
Turn the ignition to the "run" position and check the display on the OBD tool. Read the code number and reference the technical service manual for your model year of Jetta to translate the code. The technical service manual for your model year will tell you what the code means.
