Jettas manufactured in 1996 through today include an on-board diagnostic computer to monitor engine and emissions performance. You can read a trouble code from this computer by connecting a scanner to an interface plug.
The Clean Air Act of 1990 requires 1996 and newer vehicles sold in the United States to have an on-board diagnostic computer. When the OBD detects a problem, it illuminates the "check engine light" and stores a diagnostic code in its memory. You can read this code with a scanning tool.
Vehicles in compliance with the OBD-II standard have interface Data Link Connector plugs. The location of the DLC -- "check engine plug" -- varies. In many vehicles, it is beneath the dashboard on the driver's side, within a foot of the center of the vehicle.
In some imports, the interface plug is behind the ashtray or a panel on the lower left hand corner of the dashboard. Consult the Owner's Manual if the plug is not in one of these three locations. The National OBD Clearinghouse maintains a database showing plug locations by maker, model and year.
The 1997 Jetta DLC plug is behind a panel on the dashboard, right of the ashtray and above the lighter socket. Remove the ashtray completely and slide the panel left to expose the plug.
