3.2.393. SPN 521027, FMI 31, NH3 slip detectedΒΆ

Description

Too much ammonia passing through the SCR catalyst unreacted.

Possible causes are:

  • The SCR system is dosing too much DEF.
  • The DEF dosing module is defective or blocked.
  • The NOx upstream or downstream sensor is defective.
  • The SCR catalyst is defective.

Reaction in EEM

The FC is stored and the warning lamp is activated. CAN message indicate active fault.

Solution

First save the error log and then erase it. Restart the ECU after erasing the error log. If the fault occurs again during next driving cycle, check the following paths:

  • Perform a catalyst cleanup with the WinEEM service tool.
  • Check the DEF system operation. Make sure the SCR system is able to dose a correct amount of the DEF/AdBlue.
  • Check the operation with another DEF dosing module to see if the dosing module is defective.
  • Check that the NOx upstream and downstream sensors are effective. Change the defective sensors to new sensors.
  • Check the SCR catalyst condition. Remove the dosing valve and look inside the catalyst. Try to see the possible DEF/AdBlue deposit problem or blockage.