(SCR) Catalytic Converter System Test (Generation 1: M57/N57 Engine In E7X, E90)

The SCR catalytic converter converts nitrogen monoxide and ammonia to water and nitrogen (reduction). This reduction is monitored by two nitrogen oxide sensors. The efficiency and effectiveness fault memory is stored in the engine control unit if the amount of converted nitrogen oxide is insufficient. It is not always possible to clearly identify the fault cause using these monitoring methods.

The following problem sources can lead to this fault:

• Poor AdBlue quality.
• The metering module is dirty or faulty.
• The exhaust system has not been installed correctly, is leaking or faulty. The mixer has been installed incorrectly.
• Nitrogen oxide sensors faulty.
• SCR catalytic converter faulty.

The SCR catalytic converter system test is suitable to check the operability of the nitrogen oxide sensors and the SCR catalytic converter. The system test consists of 3 parts and takes approximately 1 hour:

1. Checks before the system test: 

   The following checks are required to exclude other fault causes:

   • All fault memories relevant to the SCR system must be processed.
   • The AdBlue active tank must be filled.
   • Check metering module for contamination. The three nozzles on the metering module must be free from contamination. Remove contamination with water. Run a metering quantity test using the diagnosis system.
   • Check the AdBlue quality with the refractometer.
   • Check whether the mixer has been installed correctly and is free from contamination.
2. Initialize system check: 

   The vehicle must be initialized using the diagnosis system before driving. This deactivates metering.

3. Performing a conditioning run: 

   The run is required to enable the SCR system and empty the SCR catalytic converter. Nitrogen oxide is no longer converted and the nitrogen oxide concentration is identical on the front and rear nitrogen oxide sensor.

   The run establishes the following preconditions:

   • Both nitrogen oxide sensors are enabled. For this purpose, the exhaust temperature at the SCR catalytic converter must be approximately 150 °C. The exhaust system must be free from moisture. The components are enabled after having driven continuously for approximately 15 minutes. The SCR system is fully active and monitored only after having been enabled.
   • The SCR catalytic converter is emptied.
   • Nitrogen oxide emissions are no longer converted. As a result, it is possible to check nitrogen oxide sensors in the subsequent test.
4. Carry out system test: 

   The system test includes the following checks:

   • The nitrogen oxide sensors are compared to each other. The nitrogen oxide sensors are OK if the measured nitrogen oxide concentration is identical.
   • Reactivating metering.
   • The total nitrogen oxide is determined. The check specifies whether or not the SCR catalytic converter still operates to a sufficient degree.
     NOTE: Do not switch off the ignition between conditioning run and system test.
     After the conditioning run, connect the motor vehicle to the diagnosis system when the engine is running. Metering is once again re- activated upon operating the ignition. System test unavailable when metering is switched on!