P20EF SCR NOx Pre Catalyst Efficiency Below Threshold Bank 1

Definition: This diagnostic trouble code (DTC) indicates that the Selective Catalytic Reduction (SCR) system’s efficiency in reducing nitrogen oxides (NOx) emissions before the catalyst is below the threshold for bank 1. The SCR system is not performing as expected, which may lead to increased NOx emissions.

p20ef – SCR NOx Pre Catalyst Efficiency Below Threshold Bank 1: Moderate

Common Causes

p20ef SCR NOx Pre-Catalyst Efficiency Below Threshold Bank 1

Common Causes:

• Faulty Selective Catalytic Reduction (SCR) system
• Defective NOx sensor
• Exhaust leaks before the SCR catalyst
• Poor quality diesel exhaust fluid (DEF)
• Clogged or contaminated SCR catalyst
• Software issues in the engine control module (ECM)

 

Common Misdiagnosis

OBD2 Code: P20EF

Description: SCR NOx Pre Catalyst Efficiency Below Threshold Bank 1

Common Misdiagnosis: Replacing the NOx sensor without verifying if there is an issue with the SCR catalyst or exhaust leaks.

 

Common Diagnostic Steps

OBD2 Code: P20EF

Description: SCR NOx Pre Catalyst Efficiency Below Threshold Bank 1

Common Diagnostic Steps:

• Verify the code using an OBD-II scanner and check for any additional codes.
• Inspect the Selective Catalytic Reduction (SCR) system components for damage or leaks, including hoses and connections.
• Check the Diesel Exhaust Fluid (DEF) level and quality; refill or replace if necessary.
• Examine the NOx sensors for proper operation and clean or replace them if needed.
• Test the SCR catalyst efficiency using a scan tool to ensure it is functioning correctly.
• Inspect wiring and connectors related to the SCR system for corrosion, damage, or disconnection.
• If equipped, update the vehicle’s Engine Control Module (ECM) software to the latest version available from the manufacturer.
• If all else fails, consider replacing the SCR catalyst after verifying that other components are functioning properly.