Description of On-Board Diagnostics: Diagnostic Trouble Codes

Diagnostic Trouble Codes

Diagnostic trouble codes are required by law to be structured in a manner That is consistent with SAE standard J2012.This standard uses a letter to designate the system, and four numbers to further identify and detail the malfunction. They are commonly referred to as "P-codes" and are used in addition to the familiar 5-digit VAG code.

First digit structure is as follows;
^ Pxxxx for powertrain
^ Lxxxx for body
^ Cxxxx for chassis
^ Uxxxx for future systems

Second digit structure is:
^ P0xxx Government required codes
^ P1xxx Manufacturer codes for additional emissions system function; not required but reported to the government

Third digit structure is:
^ Px1xx measurement of air and fuel
^ Px2xx measurement of air and fuel
^ Px3xx ignition system
^ Px4xx additional emission control
^ Px5xx speed and idle regulation
^ Px6xx computer and output signals
^ Px7xx transmission

The fourth and fifth digits designate the individual components and systems. For example, consider a camshaft position sensor (or related circuitry) that has a range or performance problem. This will generate the dual codes of P0341/16725 to be stored in DTC memory.