P20EEGeneric code

P20EE Code: SCR NOx Catalyst Efficiency Below Threshold (Bank 1)

The P20EE code means the SCR catalyst is not reducing nitrogen oxides (NOx) the way it should. If it is not repaired, many vehicles limit speed or will not restart after a set number of miles. Start with the DEF quality, the DEF injector and the NOx sensors before you condemn the catalyst.

Severity
High
Can I keep driving?
Yes, but with caution
Estimated cost
$30 to $3,000
Difficulty
Hard
System
Emissions Control
Applies to
All OBD2 vehicles

Common in: Citroën, Mercedes-Benz, Peugeot

Updated Reviewed by Renzo Chacca

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What the P20EE code means

Modern diesels cut nitrogen oxides with an SCR catalyst. An injector meters urea (sold as DEF, diesel exhaust fluid, or AdBlue) into the exhaust, and the catalyst uses it to convert NOx into nitrogen and water. A NOx sensor after the catalyst checks that the system is working.

The computer compares the NOx level before and after the catalyst. If the reduction is not enough, it stores the P20EE code.

Typical causes are poor quality or diluted DEF, a urea injector plugged with crystals, a failed NOx sensor or a contaminated catalyst.

Many vehicles run a countdown once an SCR fault is confirmed. If the fault is not fixed before the countdown ends, the vehicle can limit speed or refuse to start, so do not ignore the warning messages.

P20EE code symptoms

  • Check engine light or DEF warning light on.
  • A warning message with a countdown to a restricted start.
  • Limited power.

Likely causes of the P20EE code

  1. Poor quality or contaminated DEF. Confirm it with the tests in the diagnosis section.
  2. Urea injector plugged with crystals.
  3. Failed NOx sensor. Confirm it by comparing its reading with the real value.
  4. Contaminated SCR catalyst. Condemn it only after you have ruled out the other causes.
  5. Exhaust leak ahead of the sensor. Confirm it by comparing the sensor reading with the real value.

How to diagnose the P20EE code step by step

  1. Check the DEF quality with a refractometer. It should read 32.5% urea.
  2. Run the dosing test with the scan tool.
  3. Compare the NOx sensors before and after the catalyst.
  4. Look for crystals at the injector and the line.

How to fix the P20EE code

  • Replace the DEF with certified fluid.
  • Clean or replace the urea injector.
  • Replace the failed NOx sensor.
  • Replace the catalyst if it is contaminated.

Technical reference data

  • Correct DEF concentration: 32.5% urea.
  • DEF crystallizes below -11 °C (12 °F) and degrades with heat.
  • These are general reference values, so check the manufacturer data for your vehicle.

Brand-specific notes

This code is common on diesel vehicles that use DEF or AdBlue. The cases below are the brands that show it most often.

P20EE on Chevrolet and GMC

Common on Duramax diesels. Check the DEF quality and the injector first.

P20EE on Ford

Common on Power Stroke diesels. The same order of tests applies: fluid, injector, sensors, then the catalyst.

P20EE on Ram

Common on Cummins diesels. Rule out poor DEF and plugged injectors before you replace sensors.

P20EE on Mercedes-Benz

Common on Mercedes-Benz diesels that use AdBlue.

P20EE on Volkswagen

Common on Volkswagen diesels that use AdBlue.

P20EE on Peugeot

Common on Peugeot diesels that use AdBlue.

Frequently asked questions about P20EE

Can I drive with a P20EE code?
Only for a short time. Many vehicles limit speed or block the next start after a countdown, so get it checked quickly.
Can I use water instead of DEF?
No. Water damages the system and the catalyst, and it will not reduce NOx.
Is it always the catalyst?
No. Check the DEF, the injector and the sensors first, because the catalyst is the most expensive part and the least likely cause.
How much does it cost to fix P20EE?
The range is wide, roughly $30 to $3,000. Fresh DEF or an injector cleaning is at the low end, and a new SCR catalyst is at the high end.
What does the P20EE code mean?
The P20EE code is a generic OBD2 code that means the bank 1 SCR catalyst efficiency is below the threshold. The system is not converting enough NOx into nitrogen and water.

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