18B900

The variation between the measured signal and the calculated signal of the NOx sensor upstream of the SCR catalytic converter is too high.


Possible cause:

- N37/7 (NOx control unit downstream of diesel particulate filter)
- NOx sensor upstream of SCR catalytic converter
Affected functions:

- Power generation

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Test 1: Check component N37/7 (NOx control unit downstream of diesel particulate filter).

Test 2: Check component N37/7b1 (NOx sensor downstream of diesel particulate filter).

1. Check component N37/7 (NOx control unit downstream of diesel particulate filter).
IMPORTANT NOTEFollowing replacement of the N37/7 (NOx control unit downstream of diesel particulate filter) component, SCN coding has to be performed on the N3/9 (CDI control unit) control unit.Continue with button F2

Test 1.1: Check power supply of component N37/7 (NOx control unit downstream of diesel particulate filter).

Test 1.2: Inspect CAN link between component N37/7 (NOx control unit downstream of diesel particulate filter) and component N3/9 (CDI control unit).

1.1. Check power supply of component N37/7 (NOx control unit downstream of diesel particulate filter).
Figure legend

- N37/7 (NOx control unit downstream of diesel particulate filter)
- N37/7b1 (NOx sensor downstream of diesel particulate filter)
Test prerequisite

- Battery voltage is OK.
- The fuses are OK.
Test sequence

- Switch off ignition.
- Disconnect connector at component N37/7 (NOx control unit downstream of diesel particulate filter).
- Measure direct voltage with multimeter between sockets (N37/7x1) 2 and (N37/7x1) 1
- Switch on ignition.
Specified value

- Voltage [11.0...15.0] V
Question

- Is the measurement value OK?







YES
The measurement value is OK.End of test

NO
The measurement value is not OK.Possible cause and remedy

- Line from component N37/7 (NOx control unit downstream of diesel particulate filter) to component N10/1 (Front SAM control unit with fuse and relay module)
End of test

1.2. Inspect CAN link between component N37/7 (NOx control unit downstream of diesel particulate filter) and component N3/9 (CDI control unit).
If several fault codes regarding CAN communication have occurred, the following causes of the fault are likely

- Short circuit or open circuit of CAN wires
- Voltage supply of component N37/7 (NOx control unit downstream of diesel particulate filter)
Figure legend

- N37/7 (NOx control unit downstream of diesel particulate filter)
- N37/7b1 (NOx sensor downstream of diesel particulate filter)
Test sequence

- Switch off ignition.
- Detach coupling at component N37/7 (NOx control unit downstream of diesel particulate filter).
- Detach coupling at component N37/8 (NOx control unit downstream of SCR catalytic converter).
- Measure resistance with multimeter between sockets (N37/7x1) 3 and (N37/7x1) 4
Specified value

- Resistor[55...65]ohms
Question

- Is the measurement value OK?







YES
The measurement value is OK.End of test

NO
The measurement value is not OK.Possible cause and remedy

- Check lines and connectors between components N37/7 (NOx control unit downstream of diesel particulate filter) and N3/9 (CDI control unit) for short and open circuit.
End of test

2. Check component N37/7b1 (NOx sensor downstream of diesel particulate filter).
Possible cause and remedy:Component N37/7b1 (NOx sensor downstream of diesel particulate filter) is defective.Replace component N37/7b1 (NOx sensor downstream of diesel particulate filter).Further procedure:Start teach-in processes after replacement of component N37/7b1 (NOx sensor downstream of diesel particulate filter) by pressing button F2.

Warning!Communication with ECU required.