What Is OBD-II?
OBD-II (On-Board Diagnostics, second generation) is the standardized diagnostic system built into vehicles sold in the US since the 1996 model year. It monitors emissions-related components and stores diagnostic trouble codes when a fault is detected.
In plain English
Think of OBD-II as the vehicle's built-in health monitoring system. The engine computer continuously watches sensors, actuators, and systems that affect emissions. When something drifts out of range, it stores a diagnostic trouble code (DTC), may turn on the check-engine light, and records a snapshot of conditions — data a technician can retrieve with a scan tool.
The standardization is the point: the same 16-pin diagnostic connector and the same basic code structure work across every manufacturer. A P0300 code means a misfire whether the vehicle is a sedan, a truck, or a van — even though the cause of that misfire can differ by brand.
Example
Related trouble codes
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