Modern vehicle electronic control systems “know” and monitor the operating parameters of every component. When the system sees a signal that is outside normal limits, or fails to see an expected change in a signal, it stores a Diagnostic Trouble Code (DTC). To access DTCs, technicians connect a “scan” tool to a Diagnostic Link Connector. The scan tool displays any stored codes, but that is only the beginning of a full computer diagnosis. DTCs don’t tell a technician if a particular part is bad, this is why you need a professional to diagnose and repair the vehicle correctly.