Diagnosis fault memory, ME-SFI function

The ME-SFI [ME] control unit (N3/10) checks inputs and output signals for plausibility. Here it recognizes and differentiates between the following errors:

 Faults constantly present

 Loose contact fault (only for coolant temperature sensor (B11/4) and accelerator pedal sensor (B37))

The following components and systems are monitored:

 Oxygen sensors on the left and right upstream and downstream of the catalytic converter
(G3/3, G3/4, G3/5, G3/6)

 Catalytic converter function

 Catalytic converter heating

 Secondary air injection

 Smooth running control (combustion misfire)

 Fuel system

 Control units which are linked via the CAN data bus system

Driving cycle

Systems that are not constantly monitored are only checked when a driving cycle is executed.

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A driving cycle for a test consists of:

 Engine start (cold start)

 City cycle

 Outer city cycle

 Engine switch-off

As the required operating conditions are not always given, all the tests are also not completed in one drive.

Fault recognition

The following faults and error states are detected:

 Signals above or below the limit value
(open circuits, short circuits, faulty sensor)

 Frequency of fault

 Illogical combination of signals

 Function chains with faults

 Error messages for all control units reporting via the CAN data bus system

 Fault conditions of the alternator (G2)

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Engine diagnosis indicator lamp (A1e58)

If an exhaust-related fault occurs in two consecutive driving cycles, the engine diagnosis indicator lamp in the instrument cluster (A1).

In the event of a fault which could damage the catalytic converter, the engine diagnosis indicator lamp flashes.

The indicator lamp goes out after 3 correct, successive driving cycles.

Fault storage

Faults detected are stored in the fault memory together with fault freeze frame data. If the fault is not confirmed during the following driving cycle, it is deleted. Emission-relevant faults are always entered, they have a high priority.

After 40 correct driving cycles, a stored exhaust gas relevant fault and after 80 correct driving cycles, a stored TWC damaging fault is erased from the fault memory.

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Read out the fault memory

The ME-SFI [ME] control unit is connected via the chassis CAN (CAN E) and diagnostic CAN (CAN D) with the diagnostic connector (X11/4). Stored faults can be read out and deleted with DAS or with a standard output device (generic scan tool) with "Ignition ON" or with the engine running via the diagnostic connector (SAE standard ISO 9141-2).

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The fault memory is not erased if the on-board electrical system battery (G1) is disconnected.

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Fault freeze frame data (operating conditions)

If a fault is detected, the following data are stored:

 Fault code

 Vehicle speed

 Engine speed

 Coolant temperature

 Air mass

 Engine load status

 Throttle valve angle

 Ignition angle

 Selected gear (with automatic transmission 722.9)

 Mixture formation adaptation value

 Lambda control status (enabled, not enabled, locked during fuel shutoff, locked during fault detection, without correction of oxygen sensor on the left and right downstream of catalytic converter (G3/5, G3/6))

 Environmental conditions (e.g. fuel tank level, distance in the case of fault entry, counter readings for fault frequency, purge system or air injection system active)

 Alternator voltage

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Function chains are test sequences triggered by the ME-SFI [ME] control unit, for example, for air injection, regeneration and

Consequential faults

If a faulty signal is detected and the corresponding fault code is stored, all the tests in which the signal is required as a comparative parameter, are ended. This ensures that no consequential faults are stored.

Readiness code

This code makes it possible to recognize that tests relating to fault recognition are in progress and the system is thus active.

The code is set once the following tests are completed (result of test not significant):

 Catalytic converter effect

 Purge control

 Secondary air injection

 Oxygen sensor signal and heater upstream of catalytic converter

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Emergency running

If a fault is detected, substitute values are formed for the emergency running and the engine diagnosis indicator lamp comes on.

ME-SFI [ME] control unit afterrunning

After faults are erased it is necessary to wait until the processor has completed its power-down sequence after the ignition is switched off.

The duration is adaptable (programmable) and is about 5 s.