A P20EE on the 6.7L Cummins means the SCR catalyst is not reducing nitrogen oxides efficiently enough, so the aftertreatment is not meeting its target.
What P20EE means
SCR NOx Catalyst Efficiency Below Threshold is a diagnostic trouble code the engine computer sets when the monitored value falls outside its expected range. On the 6.7L Cummins, it points to a specific area of the fuel, air, or emissions system, and reading it correctly saves you from replacing good parts.
Common causes
- Poor DEF quality or dosing problems
- Failing NOx sensors giving bad readings
- A degraded or contaminated SCR catalyst
- Upstream EGR or DPF faults skewing emissions
Symptoms you may notice
- Check-engine light and SCR warning
- Countdown to a power derate
- Stored NOx sensor or dosing codes
- Reduced performance in severe cases
How to diagnose it
Check DEF quality and the dosing system, then evaluate the upstream and downstream NOx sensors. A bad sensor can falsely report low efficiency, while a genuinely degraded catalyst reads consistently low after other faults are ruled out.
How to fix it
Correct DEF quality and dosing issues and replace faulty NOx sensors before condemning the catalyst. If the catalyst is truly degraded, it must be replaced. Repair any EGR or DPF faults that affect the emissions picture, then clear and verify.
Frequently asked questions
What does P20EE mean?
The SCR catalyst is not reducing NOx efficiently enough, which can stem from DEF quality, sensors, or a degraded catalyst.
Is it always the catalyst?
No. NOx sensor faults and DEF dosing problems commonly cause it. Rule those out before replacing the expensive catalyst.
Does it cause a derate?
Yes. Like other SCR faults, it triggers a countdown to reduced power until corrected.
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