OBD-II
On-Board Diagnostics II — a standardized vehicle diagnostic system that monitors engine performance and emissions, accessible via a universal port.
OBD-II (On-Board Diagnostics, Second Generation) is a standardized vehicle self-diagnostic and reporting system mandated by the U.S. Environmental Protection Agency. Since 1996, all cars and light trucks sold in the United States must be equipped with OBD-II systems. The standard provides a universal diagnostic connector, a standard set of diagnostic trouble codes (DTCs), and a common communication protocol.
The OBD-II system continuously monitors virtually every emissions-related component and system in the vehicle, including the catalytic converter, oxygen sensors, evaporative emissions system, fuel system, engine misfires, and more. When a fault is detected, the system illuminates the Malfunction Indicator Lamp (MIL) — commonly known as the check engine light — and stores a diagnostic trouble code that identifies the nature and location of the problem.
The standard OBD-II diagnostic connector is a 16-pin trapezoid-shaped port, typically located under the dashboard on the driver's side. Any OBD-II compliant scan tool can connect to this port to read codes, view live sensor data, and access readiness monitor status. This standardization ensures that any mechanic with the proper tools can diagnose any OBD-II compliant vehicle, regardless of manufacturer.
OBD-II defines five standardized communication protocols (SAE J1850 PWM, SAE J1850 VPW, ISO 9141-2, ISO 14230 KWP2000, and ISO 15765 CAN). Since 2008, all vehicles use the CAN protocol exclusively. Modern vehicles also support enhanced manufacturer-specific diagnostics beyond the OBD-II standard, which require specialized dealer-level scan tools to access.