P20EE: Diesel NOx catalyst not removing enough NOx
SCR NOx catalyst efficiency below threshold (bank 1)
Severity: Inspect soon
P20EE means the diesel exhaust fluid system is not cutting NOx enough, and FleetRabbit says trucks can derate or be speed-limited if it is left alone.
It gets more serious if:
- Stop: Treat it as urgent if the truck loses power or goes into a speed limit, because it may be limited to a low speed.
P20EE in short
P20EE means the selective catalytic reduction system is not converting enough NOx, as measured by the NOx sensors. Poor or low diesel exhaust fluid, faulty dosing or a NOx sensor is more likely than a failed catalyst. Get it checked soon, because the engine can derate or limit speed.
Diesel trucks and cars inject diesel exhaust fluid (DEF, also called AdBlue) into the exhaust. A catalyst then turns NOx into nitrogen and water. NOx sensors before and after the catalyst show how well it works.
P20EE sets when the post-catalyst NOx reading shows too little conversion. Ross-Tech says the test runs only when the catalyst is in a set temperature window and the particulate filter is not regenerating.
FleetRabbit says to rule out DEF quality and dosing first and treat the catalyst as the last suspect. Several makers also issue software updates for related SCR codes.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Low, interrupted or wrong DEF dosing
Common general cause- Why it ranks here
- FleetRabbit lists low or interrupted dosing first; Ross-Tech lists the reducing agent injector and incorrect dosing.
- What would change this
- Rises if a dosing test fails or the injector is clogged. Falls if dosing quantity is correct.
Cause 2: DEF wrong concentration or contaminated
Common general cause- Why it ranks here
- Both sources point to DEF quality; FleetRabbit says check concentration with a refractometer.
- What would change this
- Rises if concentration is out of spec. Falls if it reads right.
Cause 3: Faulty NOx sensor or exhaust leak upstream of the catalyst
Common general cause- Why it ranks here
- Ross-Tech lists NOx sensors and exhaust leaks upstream, and says to deal with NOx sensor codes first.
- What would change this
- Rises if NOx sensor codes are stored or readings are implausible. Falls if both read sensibly and there is no leak.
Cause 4: Particulate filter condition or excess hydrocarbon dosing
Possible — needs testing- Why it ranks here
- FleetRabbit lists DPF condition and excess hydrocarbon dosing after DEF causes.
- What would change this
- Rises if the DPF has related codes. Falls if the DPF checks out.
Cause 5: Degraded SCR catalyst
Possible — needs testing- Why it ranks here
- FleetRabbit calls it the last suspect, and Ross-Tech says to replace it only if efficiency cannot be restored.
- What would change this
- Rises only after fluid, dosing, sensors and software are ruled out.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Check the DEF level and have its concentration tested.
IfConcentration is wrong or the fluid is contaminated
It meansDrain and refill the system, then retest.
IfDEF is correct
It meansTest dosing.
Step 2: Have a shop run a dosing quantity test with a scan tool and check for NOx sensor codes and software updates.
Warning:The exhaust and SCR parts are hot; DEF is also corrosive to some metals. Leave this to a shop.
IfDosing is low or the injector fails
It meansFix the dosing system.
IfNOx sensor codes or implausible readings
It meansAddress the sensors.
Points to: Faulty NOx sensor or exhaust leak upstream of the catalyst
IfAll pass
It meansCheck the DPF, exhaust leaks and then the catalyst.
Symptoms
- Check engine light and a DEF or emissions warning
- Engine derate or speed limit Gets more serious: stop and have it checked.
How to fix
Step 1: Check that the DEF tank has fluid and that the cap is on correctly.
On your vehicle
We don't have reviewed vehicle-specific evidence for P20EE yet. The causes and tests above apply across vehicles.
Related codes
No curated relationships for P20EE yet.
P20EE questions
Should I replace the SCR catalyst?
Not first. DEF quality, dosing and NOx sensors cause this code more often than a failed catalyst.
Sources and review
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error