A DTC, short for Diagnostic Trouble Code, is an alphanumeric code used to diagnose malfunctions in a vehicle or heavy equipment reported by the OBD system of your vehicle.

While the malfunction indicator lamp (MIL) also known as the check engine light - simply alerts drivers that there is an issue, a DTC identifies what and where the issue is. DTCs are also called engine vehicle fault codes codes, and can be read with a scanner that plugs directly into the port of a vehicle. Different DTCs indicate distinct vehicle problems. For example, DTC P0171 indicates a sub-optimal air-fuel mixture, whereas DTC C1446 indicates a brake switch circuit failure.

Having a basic understanding of DTCs can be helpful for drivers and fleet managers.