On-Board Diagnostics (OBD) is a system in vehicles that monitors their performance and emissions. It uses sensors to collect data about the engine, transmission, and other critical components. This information helps identify issues, ensuring the vehicle runs efficiently and meets environmental standards. When a problem is detected, the OBD system triggers a warning light on the dashboard and stores a diagnostic trouble code (DTC). Mechanics can use specialized tools to read these codes, making it easier to diagnose and fix issues, ultimately improving vehicle reliability and reducing harmful emissions.