OBD fault codes

OBD:
The theory of OBD is described on the page OBD, OBD II, EOBD. Every fault code that is read out with diagnostic software from, among others, the engine management system and displayed on the screen, has its own specific meaning. For error code P0102 (mass air flow sensor signal too weak) the malfunction needs to be traced towards the mass air flow sensor (defective MAF, fault in the plug or wiring), and for error code P02A9 it indicates that the injector of cylinder 4 is leaking.

With the fault codes, a direction is given to be able to investigate the fault. One should not immediately assume that a fault code is the actual cause of the problem. It may happen that a fault is stored relating to the lambda sensor (P0130, Oxygen Lambda Sensor bank 1, sensor 1) while the root cause is a vacuum leak in the inlet of the engine (for example, a torn crankcase ventilation hose). Too much oxygen is measured in the exhaust after combustion, causing the lambda sensor to generate a fault.
This shows that although the fault code points towards a possible direction, the expertise of the technician becomes all the more important. Replacing the crankcase ventilation hose would be the solution. Replacing the lambda sensor will not resolve the cause, and the fault will return.

There is a list with very many different fault codes available that relate to all cars. Each brand also uses its own specific codes (P1xxx). The general list is shown on this page. Especially for engine faults (where also the MIL, the Malfunction Indicator Light, is involved) there is a good chance the code with its explanation is included in this list.

TIP! Many possible causes and solutions of fault codes can be found on the site GerritSpeek.nl. There are also many tips and tricks here that can be carried out with diagnostic software. Mr. Speek is specialized in the VCDS (VAG-com) diagnostic software and has collected a vast amount of information on his site over the years.