Diagnostic Readout Box: Description and Operation

California Models Only

Diagnostic Module, N59:

The diagnostic module (N59) is located under the dashboard to the left of the steering column. Voltage is supplied from the overvoltage protection relay module (K1/2). The diagnostic module monitors emissions-related signals and systems, and in cases of malfunction, illuminates the CHECK ENGINE malfunction indicator lamp in the instrument cluster. Simultaneously, a diagnostic trouble code (DTC) is stored in the diagnostic module's memory.

The diagnostic module receives all data required for monitoring purposes via Controller Area Network (CAN) from the following control modules:
- Engine control module (N3/4),
- Electronic accelerator control module (N4/1) or cruise control/idle speed control module (N4/3).

NOTE: The retained diagnostic trouble code (DTC) memory feature of the diagnostic module has been replaced with DTC memory which is cleared after disconnecting the vehicle's battery (DM voltage supply).

In addition, the DTC readout "1" (no malfunction in system) does not appear after clearing the DTC memory (disconnecting the vehicle's battery).

DTC readout "1" only reappears during the vehicle's subsequent trip after the diagnostic module has confirmed that all monitored systems and their respective components are ok (no malfunctions).

Diagnostic Module Pressure Sensor

Diagnostic Module Pressure Sensor:

The diagnostic module pressure sensor (B5/2) senses absolute pressure in the intake manifold, converts the pressure value into a voltage signal and sends it to the diagnostic module.

CHECK ENGINE Malfunction Indicator Lamp:

Absolute pressure (Pabs) at 5V voltage supply (in mbar).


CHECK ENGINE Malfunction Indicator Lamp





The CHECK ENGINE malfunction indicator lamp (A1e26) is supplied with voltage directly from the overvoltage protection relay module (K1/2) and is illuminated via a ground signal from the diagnostic module. As a functional check, the malfunction indicator lamp illuminates for a max. of 30 seconds when switching ON the ignition and up to an engine speed of approx. 500 rpm. If there are no malfunctions, the diagnostic module opens the ground signal to the lamp and the lamp goes out.


Diagnostic Trouble Code (DTC) Memory Function

Diagnostic Trouble Code (DTC) Memory:

The diagnostic module is equipped with DTC memory that stores DTC's from monitored systems. The memory is cleared once the vehicle's battery is disconnected. With the ignition switched ON, data transmitted over CAN is received by the diagnostic module. If a malfunction should occur while driving, e.g. in the fuel injection system, the engine control module switches to a fixed operating mode, notes the malfunction and "notifies" the diagnostic module.

Diagnostic Connector X11/4:

Should the same malfunction occur during two consecutive trips, the respective DTC is stored in memory after the engine is turned OFF. The CHECK ENGINE malfunction indicator lamp illuminates with the engine running until the malfunction is repaired and the DTC is erased from memory. The DTC can be retrieved with an impulse counter scan tool at the [1][2]diagnostic connector (X11/4) or via the diagnostic module test connection pushbutton (X11/21s1).

If the malfunction only occurred once during a trip, it will be erased during the next trip.