The P20EF: SCR NOx Pre-Catalyst Efficiency Below Threshold Bank 1 fault code indicates that the SCR (Selective Catalytic Reduction) NOx pre-catalyst in Bank 1 is not performing efficiently. This reduced efficiency affects the system’s ability to reduce NOx emissions, leading to potential emissions control issues. The problem may be caused by a faulty pre-catalyst or issues within the SCR system itself.
P20EF – Quick Overview
Code | Information |
---|---|
Meaning | P20EF: SCR NOx Pre-Catalyst Efficiency Below Threshold Bank 1 |
Is it serious? | Yes, this can lead to emissions control issues and may affect overall engine performance. |
Possible causes | – Reduced efficiency of SCR NOx pre-catalyst for Bank 1 – Faulty pre-catalyst – Issues with the SCR system |
How to diagnose? | – Test the efficiency of the SCR NOx pre-catalyst – Inspect the SCR system – Check for faults in the pre-catalyst |
P20EF Meaning
The P20EF fault code means that the SCR NOx pre-catalyst in Bank 1 is not performing as efficiently as expected. The pre-catalyst plays a crucial role in reducing NOx emissions before the main SCR catalyst processes them. If the pre-catalyst is not functioning correctly, the system may fail to meet emissions standards. This issue could be caused by a failing pre-catalyst, system malfunctions, or other components within the SCR system.
Step-by-step diagnostic guide
Action | Description | Tools Needed |
---|---|---|
Check for Other Codes | Use an OBD-II scanner to check for additional codes related to the SCR system or pre-catalyst, which may provide further insight. | OBD-II Scanner |
Test the Efficiency of the SCR NOx Pre-Catalyst | Use a diagnostic tool to test the efficiency of the SCR NOx pre-catalyst by monitoring NOx emissions before and after the pre-catalyst. | Diagnostic Tool, NOx Tester |
Inspect the SCR System | Visually inspect the entire SCR system for any blockages, damage, or malfunctions that may be contributing to reduced catalyst efficiency. | Flashlight, Diagnostic Tool |
Check for Faults in the Pre-Catalyst | Inspect the pre-catalyst for damage, such as cracking or clogging, that may reduce its ability to function effectively. | Flashlight, Endoscope |
Repair or Replace Faulty Components | If the pre-catalyst or any SCR system components are found to be faulty, repair or replace them to restore proper function. | Replacement Pre-Catalyst, Tools |
Clear the Code and Test Drive | After repairs are complete, clear the code using an OBD-II scanner and test drive the vehicle to ensure the problem is resolved. | OBD-II Scanner, Vehicle Manual |
Recheck for Codes | After the test drive, rescan the vehicle to ensure the P20EF code does not return. | OBD-II Scanner |

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Hi, I am Juraj “Yuri” Lukacko. I got frustrated by unhelpful and scammy mechanics, so I decided to learn everything about car diagnostics myself. I test dozens of new car diagnostic tools every month along with learning new strategies to fix and customize cars. About Juraj Lukacko (Yuri)