P20EE: SCR NOx Catalyst Efficiency Below Threshold (Bank 1)
The SCR system isn't reducing NOx enough. The NOx sensor after the catalyst is seeing too much. One of the most common diesel emissions codes.
Green, yellow, red: three ways P20EE can look
The same code can be a quick fix or a reason to pull over. Match your situation to the closest example.
Keep an eye on it
Light on with no countdown, cleared after fresh DEF and highway driving.
Schedule a repair
Code returns or a countdown message appears. Get it diagnosed soon. NOx sensors and dosers are the usual suspects.
Stop and get help
Derated to low speed or a countdown near zero. Stop and get help. The truck may limit itself to 5 mph.
Symptoms you might notice
- Check engine light
- DEF warning
- Derate countdown if it continues
Common causes, cheapest first
- Bad, diluted or old DEF
- Crystallized or clogged DEF doser
- Failing NOx sensor (very common)
- Weak DEF pump
- Exhaust leak
- SCR catalyst contaminated
What you can check before calling
- Check DEF level and that it's fresh, sealed DEF
- Look for white crystal buildup around the doser
- Write down any countdown message
A code points to a system, not a part. Replacing parts based on the code alone is how people spend money without fixing the problem. Testing confirms the cause first.
Want the full story? Read our guide: DEF warning lights and derate: what diesel drivers need to know.
Generic codes (second character 0) mean the same thing on every vehicle. Some manufacturers add their own details, so check service information for your exact vehicle.
Related codes
More diesel & cummins codes
- P2002
Diesel Particulate Filter Efficiency Below Threshold (Bank 1) - P2463
Diesel Particulate Filter Restriction - Soot Accumulation - P242F
Diesel Particulate Filter Restriction - Ash Accumulation - P2459
Diesel Particulate Filter Regeneration Frequency - P2453
Diesel Particulate Filter Pressure Sensor A Circuit Range/Performance - P2454
Diesel Particulate Filter Pressure Sensor A Circuit Low
Look up another code
Cummins fault code (like 1922), J1939 SPN and FMI (like SPN 3251 FMI 0), or an OBD-II code (like P0420).
Questions
What does P20EE mean?
P20EE means "SCR NOx Catalyst Efficiency Below Threshold (Bank 1)". The SCR system isn't reducing NOx enough. The NOx sensor after the catalyst is seeing too much. One of the most common diesel emissions codes.
Can I drive with code P20EE?
It depends on what the vehicle is doing. Green: Light on with no countdown, cleared after fresh DEF and highway driving. Yellow: Code returns or a countdown message appears. Get it diagnosed soon. NOx sensors and dosers are the usual suspects. Red: Derated to low speed or a countdown near zero. Stop and get help. The truck may limit itself to 5 mph.
What causes P20EE?
The most common causes are: bad, diluted or old DEF; crystallized or clogged DEF doser; failing NOx sensor (very common).
Is P20EE a serious code?
It can be. Derated to low speed or a countdown near zero. Stop and get help. The truck may limit itself to 5 mph.
Skip the shop. We'll come to you.
Book online in about a minute, or call or text the office. We'll confirm the details and get a mechanic headed your way.