OBD2 is the standard on-board diagnostic system in modern vehicles. The engine control unit logs faults as trouble codes (such as P0171 or P0420), and a scan tool plugged into the OBD port reads them along with live engine data.

This page collects our OBD2 guides: individual code explanations (P0102, P0125, P0129, P0138, P0171, P0420, P0507, P059F and more), brand-specific code guides for Ford and Opel, winter-related codes, live-data checks, and notes on scan tools and VAG security gateway (SFD/SGW) access.

A trouble code does not tell you which part to replace. It only shows where the control unit noticed a problem. That is why each guide goes beyond the code itself: what to check first, which parameters to watch, and when the real cause is somewhere else. The articles are written for roadside and mobile diagnostics, based on real cases.