Diagnosing the Selective Catalyst Reduction System

The Check Engine MIL may be illuminated and DTC P20EE (NOx Catalyst Efficiency Below Threshold) may be set on some 2017-2018 Silverado and Sierra models equipped with the 6.6L Duramax diesel engine (RPO L5P). There may be several reasons why DTC P20EE has set, even when the Selective Catalyst Reduction (SCR) system is functioning properly.


The SCR system reduces oxides of nitrogen (NOx) emissions by injecting a metered amount of Diesel Exhaust Fluid (DEF), or reductant, into the exhaust gas stream entering the SCR. Within the Selective Catalytic Reduction Catalyst, the reductant reacts with the NOx to form nitrogen, carbon dioxide, and water vapor. Upstream and downstream NOx sensors provide the Engine Control Module (ECM) with engine-out and tailpipe-out NOx levels.


Check the following items to help determine if the vehicle’s SCR system is operating correctly.


Check for other DTCs

  • Complete repairs as needed for any other DTCs.
  • Clear all DTCs.


Verify DEF (or Reductant) Concentration

  • Using a scan tool, check the reductant concentration in the reductant system data.
  • The reductant concentration temperature reading may be inaccurate if the reductant is frozen or contains ice crystals. If the reductant temperature is below 25 degrees F (–4 degrees C), warm the fluid in the tank (Fig. 9) by parking the truck in a warm environment until the DEF tank temperature is warmer than 25 degrees F (–4 degrees C).
  • If the concentration is not between 28.8% and 36.2%, replace the DEF and continue with the following steps.


Fig. 9


Run the Emission Reduction Fluid Injector Quantity Test

  • If the test does not pass, refer to Emission Reduction Fluid Injector Quantity Test in the appropriate Service Information and then re-run the test.


Perform the Reductant System Malfunction Warning Service Bay Test

  • Warm the vehicle to proper operating temperature; otherwise the Reductant System Malfunction Warning Service Bay Test may time out.
  • If the test passes, return the vehicle to the customer.
  • If the test fails, repeat the Reductant System Malfunction Warning Service Bay Test up to 2 more times. If the test fails after 3 attempts, refer to the DTC P20EE service procedure in the Service Information.


For additional information, refer to #PIP5510B in the Service Information.


– Thanks to John Stempnik


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