Function Sequence For On-Board Diagnosis (OBD)

Function Sequence for Fault Detection

The ECM checks itself and its input and output signals for accuracy and detects possible malfunctions.

Malfunctions and the way they are stored are classified as follows:

Malfunction permanently present
Loose contact that occurred while driving

The following malfunctions are recognized in their frequency and duration:

Signals above or below a limit value (for example, open circuit, short circuit, sensor malfunction)
Illogical combination of various signals
Closed-loop control circuit (e.g. lambda control) at lower or upper limit of regulation interval
Malfunctions in function chains (faulty test processes, e.g. for purging)
Malfunction messages via the CAN data buses

Function Sequence for Test Procedure

The component testing and function chain testing are differentiated on test procedures.

Component test

Component testing is direct testing of a component. It includes:
- Monitoring of the power supply and electric circuits
- Comparison of the sensor signals with other sensor signals and stored comparative values
The following three test results can occur:
- Signal present (test passed)
- Signal not present (malfunction)
- Signal present, but implausible (malfunction)

Function chain test

The function chain test is indirect testing of the effect of a controlled change.
In this process individual components and systems are checked which cannot be tested by means of component testing.

The function chain is a controlled process studying cause and effect.

The ECM controls one or more components (cause) and evaluates the resulting sensor signals (effect). In the process the ECM compares the sensor signals with stored comparative values and thus recognizes trouble-free or faulty functioning of components and systems.
The following are monitored by means of function chain tests:
- Self adaptation of mixture formation
- Smooth running analysis (recognition of combustion misfire)
- Catalytic converter function
- Oxygen sensor (aging and regulation)
- Oxygen sensor heater
- Purge control

Function Sequence for Cyclic Monitoring

Cyclic monitoring takes place for components and system which are not permanently active. Regeneration, for example, only takes place when the vehicle is being operated in partial-load range and can therefore only be monitored during this operating phase.

The following components and systems are monitored cyclically:

Catalytic converter function
Catalytic converter heating
Oxygen sensor (aging and regulation)
Oxygen sensor heater
Purge control

Function Sequence for Continuous Monitoring

Continuous monitoring means constant monitoring from engine start up to "ignition OFF".

The following components and systems are monitored continuously:

Smooth running analysis (recognition of combustion misfire)
Self adaptation of mixture formation

Function Sequence for Readiness Code

In order to gain reliable information as to the trouble-free status of cyclically monitored components and systems when reading out the malfunctioning memory, these components and systems must be test ready.

The test readiness of a component or a system is shown by the readiness code. The readiness code indicates whether the malfunction detecting test is performed at least once or the component or the system is activated.

Test readiness is checked at least once per driving cycle. If test readiness exists, the readiness code is set. In order to set the components of the system at least once.

The test result is not significant in setting the readiness code. This means that it is also set if a fault in the system or the component is found.

The readiness code is set for the following components and systems once they are tested:

Catalytic converter function
Catalytic converter heating
Oxygen sensor (aging and regulation)
Oxygen sensor heater
Purge control

If test readiness does not exist for individual systems or components, it can be established using the diagnostic unit (e.g. CONSULT). To do this the function chain process is started manually using a menu item in the diagnostic software.

NOTE: All readiness codes are reset automatically when the error code is deleted.

Function Sequence for Fault Storage

Emissions-relevant malfunctions just found from the current and previous driving cycle are temporarily stored in the OBD until confirmed (through occurrence in two consecutive driving cycles) in the form of a error code, also called a DTC (diagnostic trouble code).

When an established fault occurs in two consecutive driving cycles the error code is stored in the malfunctioning memory of the ECM after ending the second driving cycle.

Driving cycle

A driving cycle consists of an engine start, vehicle journey and stopping the engine, whereby an increase in coolant temperature by at least 22 deg.C (71 deg.F) up to at least 70 deg.C (158 deg.F) must occur during travel.

Function Sequence for Avoiding Consequential Faults

When a error signal is detected and shorted, all tests where this signal is required as a reference parameter are aborted (interlock). This prevents consequential errors from being stored.

Function Sequence for Saving the Error Freeze Frame Data

Besides the malfunctions, the operating conditions under which they occur are also stored (error freeze frame data).

When the malfunction occurs second time, the associated error freeze frame data is stored again. When the malfunction continues to occur then the last stored error freeze frame data from the first and last occurrence of a malfunction can be read out.

For error freeze frame data items, refer to CONSULT Function .

Function Sequence for Fault Display

MIL (Malfunction Indicator Lamp) in the instrument cluster is actuated by the ECM via CAN communication.

When a malfunction occurs in two driving cycles, one after the other, MIL lights up.

In case of catalytic converter damage is caused by ignition misfires, MIL flashes during (remaining) driving cycle.

Fault message by means of MIL cases automatically after 3 consecutive trouble-free driving cycles.

Function sequence for Reading Out Fault Memory

Stored error codes and their error freeze frame data as well as the readiness codes can be read out with the ignition ON or engine running using CONSULT or a commercially available diagnosis tool.

Function Sequence for Erasing Faults

The system automatically clear any stored malfunctions from the fault memory only after 40 consecutive trouble-free driving cycles have occurred. They can, however, also be deleted (after repair work has been done) using commercially available diagnostic equipment or the CONSULT.