P20EE: How to Diagnose & Fix — SCR NOx Catalyst Efficiency Below Threshold
27 vehicles · 11 makes · $200-$4,500 repair
How to diagnose and fix P20EE: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
RAM 2500 · 2013–2025
The 2013+ RAM 2500 6.7L Cummins diesel DEF (Diesel Exhaust Fluid) system is plagued with failures of the DEF heater, DEF pump, DEF injector, NOx sensors, and SCR catalyst. The DEF system is required for emissions compliance, and when it fails, the truck enters a "limp mode" that limits speed to 5 mph after a countdown timer expires. DEF heater failures are most common in cold climates where the fluid freezes and cracks the heater element. TSB 18-018-20 addresses DEF heater and quality sensor issues. The NOx sensors (inlet and outlet) fail frequently at $300-500 each.
Common Symptoms
- DEF system warning light or message
- Speed limited to 5 mph (limp mode)
- Check engine light with SCR-related codes (P20EE, P2BAD, P2201)
- DEF quality poor message despite fresh fluid
- DEF tank not reading correct level
- Truck will not restart after shutdown (DEF lockout)
- Ammonia smell from exhaust
How to Fix
Diagnose with Cummins INSITE or wiTECH to identify the specific DEF component failure. Common repairs: DEF heater replacement ($400-800, Mopar 68322877AA), DEF pump/injector assembly ($600-1,000), NOx sensor replacement ($300-500 each, two sensors). If SCR catalyst is failed, replacement costs $2,000-3,500. Check TSB 18-018-20 for updated DEF heater and quality sensor procedures. Use only API-certified DEF fluid (ISO 22241 standard). Avoid storing DEF over 1 year as it degrades.
Sources for this issue (5)
Mercedes-Benz Sprinter · 2014–2025
The Sprinter diesel exhaust fluid (DEF/AdBlue) selective catalytic reduction system is plagued by faults in the dosing module, NOx sensors, DEF heater, and SCR catalyst. Failures trigger the dreaded "engine derate" countdown that limits vehicle speed to 5 mph if not resolved, stranding commercial vehicles. The system is sensitive to DEF quality and cold weather.
Common Symptoms
- Check engine light with SCR fault codes
- Engine derate countdown warning on dashboard
- Vehicle limited to 5 mph (derated)
- DEF warning light illuminated
- Higher fuel consumption
How to Fix
Diagnose the specific failed component with Star Diagnostics — the system has multiple failure points. Common repairs include the DEF dosing module, NOx sensors, DEF heater element, and DEF quality sensor. Use only ISO 22241-certified DEF fluid. A full SCR reset with Star Diagnostics is required after component replacement.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- PEAK BlueDEF ISO 22241 diesel exhaust fluid, 2.5 gallonsOEMPEAK DEF002 / ISO 22241-1
Low-level refill or drain/refill after testing confirms the DEF itself is low, contaminated or out of specification Fitment/availability: The inspected direct U.S.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Owner tips & cautions
- WarningNever use non-certified DEF fluid. Cheap DEF causes crystallization in the dosing module and destroys the SCR catalyst. Buy from reputable suppliers only. BlueDEF from Peak is widely recommended.
Ford F-250 Super Duty · 2015–2022
The Selective Catalytic Reduction (SCR) system on 2015-2022 6.7L Power Stroke trucks - which uses Diesel Exhaust Fluid (DEF) to reduce NOx emissions - is prone to failure, most commonly triggering DTC codes P207F (Reductant Quality Performance) and P20EE (SCR NOx Catalyst Efficiency Below Threshold). Causes include degraded or crystallized DEF fluid (DEF has a shelf life of 1-2 years and degrades faster in heat), a failed DEF injector, SCR catalyst failure, DEF quality sensor failure, or NOx sensor failure. In limp mode, the truck's performance is severely limited. The system is complex and diagnosis requires a professional scan tool. The DEF injector is a particularly common failure point on 2017+ trucks.
Common Symptoms
- Check engine light with P207F (Reductant Quality Performance) code
- Check engine light with P20EE (SCR NOx Catalyst Efficiency Below Threshold)
- Reduced engine power / limp mode
- DEF gauge or warning light showing issue
- Truck will not restart after excessive limp mode cycles
- Rough running at idle
- Increased fuel consumption
How to Fix
Diagnose with Ford IDS or quality OBDII scanner capable of reading all PCM codes. Common fixes: (1) Drain and refill DEF tank with fresh fluid (AutoZone DEF, Ford Motorcraft DEF VC-10-B) - $15-$30. (2) Replace DEF quality sensor ($150-$300). (3) Replace DEF injector ($300-$600 + labor). (4) SCR catalyst replacement ($2,000-$4,000). Always use fresh, quality-brand DEF (avoid bargain/store-brand DEF). Do not let DEF tank run to empty repeatedly.
Sources for this issue (1)
Chevrolet Colorado · 2016–2023
GMC Canyon · 2016–2025
Find P20EE by make
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P20EE code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P20EE mean?
P20EE stands for "SCR NOx Catalyst Efficiency Below Threshold." This is a diesel emissions code meaning the Selective Catalytic Reduction (SCR) catalyst on Bank 1 is not cleaning up exhaust as well as it should. The SCR system sprays Diesel Exhaust Fluid (DEF) into the hot exhaust to convert harmful nitrogen oxides (NOx) into harmless nitrogen and water; NOx sensors before and after the catalyst tell the computer how well it is working. When the measured efficiency falls below the required threshold, this code sets. Because it is an emissions-protection code, it often triggers warning lights and can lead to reduced power or a speed limit (a 'derate') if left unfixed.
What are the most common causes of P20EE?
The most common causes of P20EE are: Poor quality, contaminated, or wrong DEF fluid, Faulty or clogged DEF injector/dosing valve, Failed upstream or downstream NOx sensor, Degraded or failed SCR catalyst, Exhaust gas temperature sensor fault affecting dosing, Exhaust or DEF line leak, DEF line/heater problems (crystallization or freezing). The specific cause varies by vehicle.
How much does it cost to fix P20EE?
Repair costs for P20EE range from $200 to $4,500, depending on the vehicle and root cause.
Which vehicles are affected by P20EE?
Au7o has documented P20EE across 27 vehicle models from 11 manufacturers: Chevrolet, Dacia, Ford, GMC, Genesis, Jeep, Mercedes-Benz, Opel, Peugeot, RAM, Skoda.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.