Degraded or contaminated selective catalytic reduction catalyst; Faulty or skewed nitrogen oxide sensor upstream or downstream of the catalyst; Incorrect diesel exhaust fluid quality, dilution, or contamination; Defective diesel exhaust fluid injector or dosing module delivering insufficient reagent; Exhaust leaks between the dosing point and the catalyst; Software calibration issue in the powertrain control module; Restricted or damaged exhaust system components affecting flow and temperature

DTC codes / P20xx · Fuel and air metering and auxiliary emission controls / P20EE

P20EENOx Catalyst Efficiency Below Threshold (Bank 1)

Q: What does code P20EE mean?

NOx Catalyst Efficiency Below Threshold (Bank 1). P20EE means the powertrain control module has determined that the selective catalytic reduction catalyst on bank 1 is not converting nitrogen oxides efficiently enough to meet the emissions target. The computer compares the nitrogen oxide sensor readings before and after the catalyst and flags the code when the downstream sensor reports too little reduction over a monitored drive cycle.

P20EE means the powertrain control module has determined that the selective catalytic reduction catalyst on bank 1 is not converting nitrogen oxides efficiently enough to meet the emissions target. The computer compares the nitrogen oxide sensor readings before and after the catalyst and flags the code when the downstream sensor reports too little reduction over a monitored drive cycle.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
A stored code with no drivability complaint in many cases
Reduced engine power or limp mode on some vehicles after repeated fault detection
Slightly rougher running or hesitation under load if the fault is severe
Increased diesel exhaust fluid consumption as the system attempts to compensate
Emissions monitor readiness not setting, preventing a passing inspection

Common causes, most likely first

Degraded or contaminated selective catalytic reduction catalyst
Faulty or skewed nitrogen oxide sensor upstream or downstream of the catalyst
Incorrect diesel exhaust fluid quality, dilution, or contamination
Defective diesel exhaust fluid injector or dosing module delivering insufficient reagent
Exhaust leaks between the dosing point and the catalyst
Software calibration issue in the powertrain control module
Restricted or damaged exhaust system components affecting flow and temperature

How to diagnose and fix it

Scan all modules and record freeze-frame data, then verify the code is current rather than history
Inspect the diesel exhaust fluid level, concentration, and contamination, and correct or replace the fluid if needed
Check for exhaust leaks and physical damage between the injector and the catalyst
Command the diesel exhaust fluid dosing system with a scan tool and confirm the injector delivers the correct quantity
Compare upstream and downstream nitrogen oxide sensor readings live to confirm catalyst efficiency and sensor accuracy
Verify the powertrain control module has the latest calibration and reprogram if a known update applies
Replace the selective catalytic reduction catalyst only after the above checks confirm it is the failed component

Why this code often is not the catalyst

A diluted or contaminated diesel exhaust fluid tank is one of the most common causes and is frequently overlooked because the fluid looks normal.
A lazy or skewed nitrogen oxide sensor can mimic a failed catalyst, so sensor readings must be verified against a known-good reference before any catalyst replacement.
Exhaust leaks upstream of the catalyst let untreated exhaust bypass the brick and set this code even when the catalyst is healthy.
The code can be set by a dosing injector that sprays but delivers the wrong quantity, which a visual inspection will never reveal.
Replacing the catalyst without correcting the root cause will usually result in the code returning within a few drive cycles.

Can I keep driving with P20EE?

The vehicle remains safe to drive in most cases, but continued operation with a failed catalyst will increase emissions and may eventually trigger power derate. Avoid long periods of heavy load or towing until the fault is diagnosed, and address it promptly to prevent secondary damage.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.