Two Data Link Connectors (DLC) are normally used. A scan tool is connected to a DLC to read the data stream, capture Diagnostic trouble Codes (DTCs), read real-time operating information and even operate some of the vehicle's systems for testing. Most all of the modern vehicles will require a scan tool for efficient troubleshooting. The Data Link Connector designated DLC1 is located in the engine compartment. The Data Link Connector designated DLC3 is located in the interior of the vehicle, under the driver's side of the instrument panel.