Diagnostic Trouble Code

Definition

A dtc/">Diagnostic Trouble Code (DTC) is an alphanumeric code generated by a vehicle's on-board computer system (obd2/">OBD-II) when it detects a malfunction in any monitored subsystem. These codes are standardized under SAE J2012 and ISO 15031-6 regulations, allowing technicians and drivers to pinpoint specific electrical, mechanical, or chemical failures.

When a sensor reports readings outside of normal operating tolerances, the PCM waits for a predefined number of drive cycles. If the fault persists, it stores the DTC in its memory and illuminates the Malfunction Indicator Lamp (MIL), commonly known as the Check Engine Light. This system prevents catastrophic mechanical damage, preserves passenger safety, and ensures the vehicle complies with environmental emission standards.

DTCs are structured with five characters indicating system (e.g. Powertrain codes starting with P, Body codes with B, Chassis with C, and Network with U). For example, misfire codes like P0300 indicate ignition faults, while EVAP system faults are labeled under P0440 or P0455. Technicians retrieve these codes using scan tools connected to the vehicle's OBD-II diagnostic port.