P20EE: SCR NOx Catalyst Efficiency Below Threshold (Bank 1)
The SCR system on bank 1 is not reducing nitrogen oxides enough.
Normal risk
Usually drivable. A restart block may follow.
Short answer · Can I keep driving with P20EE?
Usually yes. Watch the dashboard for a countdown. Have the AdBlue quality and dosing checked soon.
What does P20EE mean?
Code P20EE means the NOx sensors show that the SCR catalyst is not converting enough nitrogen oxides.
The cause can be the fluid, dosing, the NOx sensors or the SCR catalyst. Many markets require a restart block if the fault is not fixed.
What the car does when the code is set
The AdBlue or check engine light comes on. If the fault continues, many cars show a countdown and eventually block a restart.
How the system works
The SCR catalyst converts nitrogen oxides into nitrogen and water with ammonia made from AdBlue. NOx sensors before and after the catalyst measure how much NOx enters and how much leaves. From these readings the control unit calculates the conversion efficiency. If it stays below the legal threshold, the code is stored. The cause can be the fluid, the dosing, the catalyst or the sensors themselves.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Poor AdBlue quality or low dosingToo little ammonia reaches the catalyst.
- Faulty NOx sensorThe sensor reports too much NOx.
- Worn SCR catalystAgeing or contamination.
- Exhaust leaksLeaks distort sensor readings.
How it is diagnosed
- Check fluid qualityMeasure the urea concentration.
- Run a dosing testA scan tool doses into a measuring cup to check the amount.
- Compare NOx sensor readingsCheck upstream and downstream sensors.
- Check for exhaust leaksFix leaks before replacing parts.
Questions about P20EE
Does P20EE mean the SCR catalyst must be replaced?
Rarely as a first step. The fluid quality, the dosing amount and the NOx sensors are checked first, because they are more common causes and cheaper to fix.
Can short trips cause P20EE?
Yes. The SCR system needs a warm exhaust to work. Many short trips in cold weather can lower its efficiency and, together with other faults, set the code.
Will the code clear itself?
If the cause was temporary, such as poor fluid that has since been replaced, the system may pass its test again after some driving. A workshop can also run the test with a scan tool.
Is the NOx sensor often to blame?
NOx sensors are a common cause. A workshop compares readings and tests dosing before replacing anything.
Will AdBlue additives help?
Use only AdBlue meeting ISO 22241. Additives are not approved and may damage the system.