Power units
Signal use
The signal from the reducing agent quality sensor G849 is evaluated by the evaluation electronics in the delivery
unit for reducing agent metering system GX19 and transmitted to the engine control unit J623 in the form of a
SENT signal. If the urea concentration of the reducing agent is too low or too high, an entry will be made in the
engine control unit event memory. The warning for an SCR system fault is displayed in the dash panel insert.
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The urea concentration in the reducing agent may be too low if, for example, the reducing agent tank has
been filled with water or AdBlue® that does not comply with emission standards.
The urea concentration in the reducing agent can be too high if, for example, a vehicle is not driven for a very
long period and water has evaporated from the AdBlue®.
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Effects of signal failure
If the signal from the reducing agent quality sender fails, an entry will be made in the engine control unit event
memory. The AdBlue® warning indicator for a fault in the SCR system is switched on in the dash panel insert
display.
Nitrogen oxide sensors
Task
The engine control unit measures the amount of nitrogen oxide in the exhaust gas as well as the efficiency of the
SCR system using the signals from the two NOx senders.
Fitting location
The NOx sender G295 is located in the emission control module upstream of the oxidising catalytic converter and
the reduction catalytic converter. The NOx sender 2 G687 is located downstream of the ammonia trap. Both NOx
senders have a separate control unit for evaluating and transmitting the signals to the engine control unit J623.
Signal use
The signals from the two NOx senders are used to monitor the efficiency for the SCR system as part of the
European on-board diagnosis. The engine control unit compares the nitrogen oxide concentration before and
after exhaust gas treatment with the signals from the NOx sender G295 and the NOx sender 2 G687. If the
nitrogen oxide concentration in the exhaust gas deviates from the specified values in the engine control unit, the
exhaust emissions warning lamp K83 (MIL) will be switched on. The diagnosis processes for calculating the
efficiency are performed in the engine part load range.
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