On-board diagnosis, function

Function requirements for On-Board Diagnosis (OBD), general points

 Circuit 87M (engine timing ON)

On-board diagnosis, general points

An On-Board Diagnosis system of the second generation is used (OBD II)). In Europe, the OBD II, with appropriate adaptation for the European market, is called European On-Board Diagnosis (EOBD). The OBD system is integrated in the CDI control unit (N3/9) and has the following tasks:

– monitor emissions-relevant components and systems while traveling

– identify and save malfunctions

– detected faults are transmitted over a uniform interface (diagnostic connector (X11/4)) to a diagnostic unit (e.g. Xentry Diagnostics))

– display malfunction by the engine diagnosis indicator lamp icon in the IC (A1)

OBD pursues the follow objectives:

– ensuring permanently low exhaust emissions

– protect components at risk (such as catalytic converters) against backfires

The following components and systems are monitored:

– intake air path

– fuel system

– glow system

– exhaust gas recirculation

– smooth running control

– oxygen sensor

– oxygen sensor heater

– diesel particulate filter

– AdBlue® system

– other emission-relevant components or components whose malfunctioning prevents the diagnosis of another component

Function sequence for on-board diagnosis (OBD)

The OBD is described in the following steps:

 Function sequence for fault detection

 Function sequence for test procedure

 Function sequence for cyclic monitoring

 Function sequence for continuous monitoring

 Function sequence for readiness code

 Function sequence for fault storage

 Function sequence for avoiding consequential faults

 Function sequence for saving the fault freeze frame data

 Function sequence for fault display

 Function sequence for reading out fault memory

 Function sequence for clearing faults

Function sequence for fault detection

The CDI control unit checks itself and its input and output signals for plausibility 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 at lower or upper limit of regulation interval

– malfunctions in function chains (faulty test sequences,

for example for the smooth running control)

– malfunction messages via the CAN data buses

Function sequence for test procedure

For test procedures one differentiates between component testing and function chain testing.

Component testing

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 CDI control unit controls one or more components (cause) and evaluates the resulting sensor signals (effect). In the process the CDI control unit compares the sensor signals with stored comparative values and thus recognizes fault-free or faulty functioning of components and systems.

The following are monitored by means of function chain tests:

– smooth running control

– exhaust gas recirculation

– oxygen sensor heater

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:

– fuel system

– exhaust gas recirculation

– smooth running control

– oxygen sensor

– oxygen sensor heater

– regeneration

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:

– intake air path

– glow system

– particulate filter

– AdBlue® system

– crankcase ventilation system

­  cooling system

– all other emission-relevant components

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 fault 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 tells you whether malfunction detection tests have been run at least once, indicating that the component or the system is active.

Test readiness is checked at least once per driving cycle. If test readiness exists, the readiness code will be set. In order to set the readiness code it is sufficient if the vehicle has checked all of the components belonging to a 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 have been tested:

– fuel system

– exhaust gas recirculation

– smooth running control

– oxygen sensor

– oxygen sensor heater

– regeneration

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



All readiness codes are reset automatically when the fault 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 fault code, also called a diagnostic trouble code or DTC.

If a found malfunction occurs in two consecutive driving cycles, the fault code is stored in the CDI control unit fault memory after the second driving cycle is completed.

 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°C up to at least 70°C must occur during travel.

Function sequence for avoiding consequential faults

If a faulty signal is detected and stored, all tests where this signal is required as a reference parameter are aborted (interlock). This prevents consequential faults from being stored.

Function sequence for saving the fault freeze frame data

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

If the malfunction occurs a second time, the associated fault freeze frame data will again be stored. If the malfunction continues to occur then the last stored fault freeze frame data will be updated. This means that the fault freeze frame data from the first and last occurrence of a malfunction can be read out.

Fault freeze frame data include:

– vehicle speed

– engine speed

- coolant temperature

– intake air temperature

– charge air temperature and charge air pressure

– supply voltage

– engine throttle condition

– adaptation value for injection regulation

Function sequence for fault display

The engine diagnosis indicator lamp symbol in the instrument cluster () is actuated by the CDI control unit via drive train CAN (CAN C1), the powertrain control unit (N127) via the suspension FlexRay (Flex E), electronic ignition lock control unit (N73) and user interface CAN (CAN HMI). If a fault occurs consecutively in two driving cycles, the engine diagnosis indicator lamp symbol lights up. In the case of catalytic converter damaging ignition misfires, the engine diagnosis indicator lamp symbol flashes for as long as the ignition misfires occur and then lights up permanently during the whole (remaining) driving cycle.

A fault display using the engine diagnosis indicator lamp symbol goes out automatically after 3 consecutive trouble-free driving cycles.

Function sequence for reading out fault memory

The CDI control unit is connected via the drive train CAN, powertrain control unit, suspension FlexRay, electronic ignition lock control unit and diagnostic CAN (CAN D) to the diagnostic connector. Stored fault codes and their fault freeze frame data as well as the readiness codes can be read out with the ignition ON or engine running via the diagnostic connector using a commercially available diagnostic unit or Xentry Diagnostics.

Function sequence for clearing faults

The system will 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 Xentry Diagnostics system.