Most Common OBD-II Trouble Codes Explained . (Complete Guide for US Car Owners) If your Check Engine Light has turned on, an OBD-II scanner can read a diagnostic trouble code (DTC) that helps identify the problem. Since 1996, all cars sold in the United States use the OBD-II standard, meaning many common codes have the same basic definition across brands like Ford, Toyota, Honda, Chevrolet, Nissan, BMW, Mercedes-Benz, Volkswagen, and others. This guide explains the most common OBD-II codes, what they mean, common symptoms, likely causes, and links to detailed troubleshooting. What Is an OBD-II Code? OBD-II (On-Board Diagnostics II) is the standard diagnostic system used in modern vehicles. When the engine or emissions system detects a fault, it stores a trouble code and usually turns on the Check Engine Light. Most Common OBD-II Codes P0420 – Catalyst System Efficiency Below Threshold (Bank 1) Usually caused by a failing catalytic converter, a faulty oxygen sensor, or an exhaust leak....