Description of On-Board Diagnostics

OBD II:

Since 1988, motor manufacturers have been required by the California authorities (and later by the authorities of other countries around the world) to incorporate smart' electronic control systems in the car so that the occurrence of emission-related faults can be determined and located by means of OBD (On-Board Diagnostics).

For model year 1994 and later cars, the California requirements will be even stricter through the introduction of OBD II. One purpose of the new requirements is to bring about the standardization of diagnostic trouble codes (fault codes) and command codes so that different motor manufacturers will all use the same codes, which will make life easier for technicians who work on several different systems and makes of car.

Recommendations for this standardization are set forth in document SAE J-2012.

In view of the above and for practical reasons, diagnostic trouble codes for the TRIONIC SFI system from M93 are already different from those we have previously been accustomed to when obtaining and reading diagnostic trouble codes on an ISAT scan tool.

OBD II:

The new diagnostic trouble codes (DTCs) consist of two parts: the first part consists of a letter and a numeral, the second part consists of three numerals (e.g. PO 111).

From the letter we can see which system is involved, as follows:
^ P = Power train
^ C = Chassis
^ B = Body

In addition to these there is also U (Undefined) in reserve.

From the numeral or letter we can see whether the code concerns a legal requirement according to SAE (0) or whether it is a unique manufacturer code (1 or 2).

From the second numeral or letter we can see which subsystem in each main group the DTC indicates.

In Power diagnostic trouble codes, the second numeral has the following significance:

P01xx Fuel/air supply
P02xx Fuel/air supply
P03xx Ignition system
P04xx Emission control system
P05xx Engine speed/idling control
P06xx ECM and ECM output signals
P07xx Transmission
P08xx Transmission
P09xx Reserved for SAE
P00xx Reserved for SAE

The last two numerals in the DTC comprise a serial number which gives each DTC in the different groups a unique number.

Command Codes:

Changed command codes do not refer back to OBD II requirements but, somewhat simplified, could be said to be an adaptation to and/or utilization of a standard used by GM.

Apart from their new pattern, the command codes have also been grouped according to their purpose, as follows:
^ codes for handling DTCs, identification and communication (engine switched off or running)
^ codes for checking the status of components and/or functions connected to the ECM (engine running)
^ codes for activating components with the engine switched off
^ codes for activating components with the engine running

Note:
When command codes beginning with EE or EF are to be used, the ISAT scan tool's ALT key must be used to scroll through the letters E-F-G-H on the display, see Fig. above.

Intermittent Faults:

With the introduction of the new trouble codes, the earlier unique fault 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 method of indicating one has been changed and the same trouble code is used regardless of whether the fault is permanent or intermittent.

A status flag and a counter are incorporated in the control module.

The status flag keeps track of whether the fault is permanent or intermittent.

Each occurrence of a fault is added in the counter. When the total number of fault occurrences has filled the counter to capacity, its contents are frozen.

When the ignition is switched off, the contents of both the status flag and the counter are stored in a non-volatile memory.

If the fault is intermittent and the counter has not been filled to capacity, its contents are decremented by a certain number next time the ignition is switched on.

If the intermittent fault persists, the counter again starts to add each fault occurrence until it is filled to capacity. If the fault has disappeared, the contents of the counter will be retained until the next stop/start cycle when the counter will again be decremented by a certain number.

Consequently, after a number of start/stop cycles the counter will have counted down to zero and the fault (trouble code) disappears (unless fresh fault occurrences have been added).

If the fault is of such a nature that the CHECK ENGINE lamp (MIL) lights up, it will also go out when the counter decrements to zero.

Due to a time-limiting function in the control module, the above countdown cannot be initiated by carrying out a number of stop/start cycles in quick succession. At least 15 minutes must elapse between each cycle for the counter to decrement its contents.