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Diagnostic fault codes adapted to new legalrequirements

Diagnostic fault codes adapted to new legal requirements


Since 1988, motor manufacturers have been required by the authorities in California (later followed by other countries around the world) to incorporate "smart" control systems in the car for the purpose of detecting and localizing emission- related faults with the aid of OBD (On-Board Diagnostics).

From model year 1993 and later cars, the Californian requirements were further sharpened through the introduction of OBD II. In part, the new requirements entailed a standardization of the structure of fault diagnosis and command codes. This was done in order to achieve a degree of uniformity between different motor manufacturers and so make things easier for mechanics and other garage personnel who work on several different systems and makes of car. Recommendations for this standardization are to be found in document SAE J-2012.

In the light of the above background, the diagnostic trouble codes are different for the M1994 TCS system than what we have previously been accustumed to when using the ISAT scan tool for obtaining readouts of diagnostic trouble codes.

Intermittent faults

With the advent of the new diagnostic trouble codes, the previous unique code for an intermittent fault has been discontinued. This does not mean that it will no longer be possible to detect an intermittent fault, but the new method of obtaining an intermittent fault readout is different to the previous method.

The new diagnostic trouble codes consist of two parts: a first part consisting of a letter and a figure, followed by a second part consisting of three figures (e.g. P0 111).

The letter denotes system affiliation, as follows:



• P = Power train

• C = Chassis

• B = Body

Also used in addition to the above is the letter U (Undefined), which is in reserve. The figure after the letter indicates whether the code is associated with a legal requirement according to SAE (0) or if it is a unique manufacturer's code (1 or 2).

The second figure after the letter shows the subsystem in the relevant main group to which the diagnostic trouble code refers. For Power train diagnostic trouble codes, the second figure signifies the following:

P01xx Fuel/air supply

P02xx Fuel/air supply

P03xx Ignition system

P=04xx Exhaust emission control system

P05xx Speed/idling control

P06xx Control module and control module output signals

P07xx Transmission

P08xx Transmission

P09xx Reserved for SAE

P00xx Reserved for SAE

Finally, the last two figures in the diagnostic trouble code comprise a serial number which gives each diagnostic trouble code a unique number in the respective group.