P20EE
SCR NOx Catalyst Efficiency Below Threshold
Get Your FREE AI Diagnostic Report
Enter your vehicle for a personalized diagnosis for code P20EE
The P20EE trouble code — SCR NOx Catalyst Efficiency Below Threshold — indicates that your vehicle's Selective Catalytic Reduction (SCR) system is not converting nitrogen oxide (NOx) emissions at the efficiency level required by federal emissions standards. The SCR system uses Diesel Exhaust Fluid (DEF), a urea-water solution, to chemically break down harmful NOx gases in the exhaust stream. When the powertrain control module (PCM) detects that the downstream NOx concentration is too close to the upstream reading, it concludes that the catalyst is underperforming and sets this code.
The most common triggers for P20EE include low-quality or contaminated DEF fluid, a malfunctioning DEF dosing injector, a degraded or poisoned SCR catalyst, or a faulty NOx sensor providing incorrect feedback. Because diesel emissions regulations are strict, many modern trucks and SUVs equipped with SCR systems will enter a derate or limp mode if this code is left unresolved — limiting engine power and, in some cases, eventually preventing the vehicle from restarting. Early diagnosis is strongly recommended to avoid these consequences.
DIY repair is possible for straightforward causes like replacing DEF fluid or swapping a NOx sensor, but SCR catalyst replacement is labor-intensive and expensive, often requiring professional equipment and programming. Always use DEF fluid that meets ISO 22241 (API Certified) specifications, as off-brand or diluted fluid is a leading cause of this code. If the code returns after refreshing DEF, a scan tool capable of reading live NOx sensor data is essential to determine whether the root cause is the catalyst, injector, or sensor.
Severity
Symptoms
- •Illuminated check engine light (MIL)
- •Failed emissions test or inspection
- •Reduced fuel economy
- •DEF (Diesel Exhaust Fluid) warning light may appear
- •Possible entry into limp mode or power deration on some vehicles
- •Increased exhaust smoke or unusual exhaust odor
Likely Causes
The selective catalytic reduction (SCR) catalyst itself may be contaminated, physically damaged, or worn beyond its service life, preventing it from converting NOx emissions at the required efficiency threshold.
Diluted, contaminated, or incorrect DEF fluid reduces the urea concentration available for the SCR reaction, causing NOx conversion efficiency to fall below OBD threshold limits.
A malfunctioning DEF injector may deliver insufficient or inconsistent urea spray into the exhaust stream upstream of the SCR catalyst, degrading catalyst conversion performance.
A failed or skewed NOx sensor can report inaccurate efficiency readings to the ECM, triggering a false P20EE even when the SCR system is physically functioning correctly.
Exhaust leaks upstream or downstream of the SCR catalyst can skew NOx sensor readings and alter the exhaust temperature profile required for optimal catalyst operation.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Drain and refill DEF tank with fresh, OEM-specification fluid and clear codes
- Inspect and replace faulty DEF injector or dosing pump
- Replace failed upstream or downstream NOx sensor
- Clean or replace the SCR catalyst assembly
- Repair exhaust leaks upstream or downstream of the SCR converter