What is OBD-II and why is it important?

On-Board Diagnostics II (OBD-II) is a standardized system for vehicle self-diagnosis and reporting. It monitors emissions-related systems, stores diagnostic trouble codes (DTCs) when faults are detected, and provides a standard connector and communication protocols for scan tools. OBD-II is mandatory in US since 1996, EU since 2001. It enables emissions compliance verification, simplifies repairs by standardizing fault codes, and supports aftermarket diagnostic tools.