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P20EE Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


P20EE is an OBD-II Code that refers to SCR NOx Catalyst Efficiency Below Threshold


A P20EE code is a generic diagnostic trouble code that refers to the PCM monitoring the Reductant Catalyst System’s use of DEF and its ability to convert NOx to Nitrogen. Diesel Exhaust Fluid (DEF) is used to assist in converting leftover exhaust particles to harmless gases. 

The temperature of the exhaust system must rise in order to burn off those particles, and the injection of DEF Fluid into the exhaust stream raises the Diesel Particulate Filter (DPF) temperature which allows this to happen. If the Reductant Catalyst fails to perform its function correctly, code P20EE is set.


Common causes for this code include:

  • Bad Diesel Particulate Filter (DPF)
  • Leaks in Exhaust 
  • Malfunctioning NOx Sensor
  • Contamination in Fuel
  • Insufficient DEF fluid 
  • Ineffective Selective Catalytic Reduction (SCR) Catalyst


The symptoms of a P20EE code are:

  • Black smoke from vehicle exhaust in excess
  • Engine performance has decreased
  • Fuel efficiency has been reduced
  • Other stored emission and SCR related codes


To diagnose a P20EE DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure codes
  2. Review vehicle history reports for any NOx related codes previously set.
  3. Check for visible smoke from tailpipe and inspect the exhaust system, looking for leaks or damage.
  4. Check hose fittings to make sure there are no obstructions. 
  5. Inspect DPF or SCR Catalyst exterior for flame outs or apparent signs of damage.
  6. Inspect DEF fill tube for leaks, cap integrity and proper cap seal to fluid line.
  7. Check DTC status in the ECM to see if the SCR system has been enabled.
  8. Check main engine parameters for signs of damage or excessive fuel use due to an injector misfire or a drop in turbo boost.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a P20EE code:

  • Not checking the freeze frame data in the ECM
  • Not verifying vehicle history report for SCR related codes
  • Fixing one issue without fully understanding how it relates to other problems with the system.

How serious is this?

A P20EE code is considered to be serious because the SCR system failure will allow raw fuel to be dumped into the exhaust stream. This can cause problems down the line, leading to emissions failure and damage to other parts of the vehicle’s emissions system.

What repairs can fix the code?

repair manualsrepair manuals

The following are solutions that may fix this problem:

  • Replacing DEF Fluid
  • Replacing NOx Sensor
  • Replacing DPF Filter Element
  • Replacing harness related to the SCR system
  • Replacing a damaged or clogged SCR Catalyst
  • Replacing corroded or cracked terminals
  • Replacing damaged or misfiring injectors
  • Installation of a Turbocharger Boost Sensor

Related codes

A P20EE is related to and may be accompanied by the following codes:

P2031 – SCR Malfunction

P2032 – SCR System Switching Valve Malfunction

P2137 – Front Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 1 Sensor 1

P2138 – Front Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 1


In conclusion, the P20EE code is a diagnostic trouble code that refers to a malfunctioning SCR NOx Catalyst Efficiency Below Threshold. This can be caused by a number of problems, but the most common culprits are DPF Filter Element and DEF Fluid related issues. A technician should verify these potential causes and check for service manual in order to properly diagnose and repair this code.