Diagnostic Trouble Code Help Copyright AA1Car . A Diagnostic Trouble Code (DTC) is set in a vehicle's onboard computer when a fault occurs in any monitored system. The code number corresponds to the type of fault, and can be used to diagnose the problem. When an engine is running and the computer detects a problem in one of its sensor or output circuits, or even within itself, it will set a trouble code. In some systems, the trouble code number is retained in memory. In others, the trouble code is not stored but is regenerated when a mechanic runs the system through a special self-diagnostic test. A trouble code will turn on the Check Engine Light (malfunction indicator lamp or MIL) to alert the driver the vehicle has a problem. To diagnose the fault, a code reader or scan tool is connected to the vehicle diagnostic connector to read the trouble code. Some tools display a definition for the code, while others show only a trouble code number. You then have to look up the definiti ... Read more »