Selective Catalytic Reduction (SCR) Warning And Shutdown Scenario

SCR warning and shutdown scenario 

The warning and shutdown scenario is activated if the vehicle nitrogen oxide emissions are exceeded:

1. The conversion of the nitrogen oxide exhaust gas (nitrogen monoxide) to nitrogen dioxide is monitored by the two nitrogen oxide sensors upstream and downstream of the SCR catalytic converter. The nitrogen oxide levels upstream and downstream of the SCR catalytic converter are compared and a conversion rate calculated. If this rate drops below a limit value, the warning and shutdown scenario is activated.
2. The engine control unit compares the quantity of urea/water mixture metered in with the nitrogen oxide conversion. In a trouble-free system, this factor is 1. If too much urea/water mixture is required for the conversion of the nitrogen oxide exhaust gases, the factor rises. From a factor of 1.49, the warning and shutdown scenario is activated.

A Check Control message is displayed and an entry made in the fault memory. In addition to the fault code entries for the warning and shutdown scenario, further SCR faults may be entered. A residual running distance for the vehicle is displayed. When the residual running distance has been travelled, engine start is prevented if the fault is not remedied.

Possible causes of fault: 

• Urea/water mixture quality deficient, misfuelling, SCR active tank empty
• Nitrogen oxide sensors faulty (poisoned, sensor drift)
• SCR catalytic converter faulty (not enough nitrogen oxide is converted)
• Metering module faulty (too little urea/water mixture metered in)
• SCR tank system faulty (too little urea/water mixture metered in, system pressure in the metering line is too low)

Fault elimination measures: 

• Check urea/water mixture quality
• Remedy all SCR system faults
• Completely fill the SCR active tank, check the fluid level with the diagnosis system; if necessary, activate refuelling detection with the corresponding service function
• NOTE: reset long-term adaptation. Carry out the service function "SCR adjustments and adaptations". Select the menu item "Diesel exhaust fluid changed, reset wrong media. (long-term adaptation factor)" in the service function.
• Carry out a conditioning run  with the following driving profile:
  • If the residual running distance is less than 90 km (55 miles). the residual running distance can be extended once with the service function SCR reset.
  • Drive for one hour at constant speed with phases of slight engine traction and at a speed of approx. 70 - 90 km/h.
  • If conversion of the nitrogen oxide exhaust gases can be performed successfully, the Check Control message on the instrument cluster goes out. The warning and shutdown scenario faults have been remedied and can then be deleted.
  • NOTE: The driving time is one hour, both nitrogen oxide sensors must be enabled. Only then is a system check performed. The enabling can be checked with the diagnosis system. Depending on the exhaust temperature, the nitrogen oxide sensors are enabled within 15-35 minutes.
• Delete fault memory with the diagnosis system.