P20EE on Mercedes-Benz
SCR NOx Catalyst Efficiency Below Threshold
P20EE on Mercedes-Benz vehicles indicates scr nox catalyst efficiency below threshold. Au7o has documented this code across 4 Mercedes-Benz models — most commonly on GLE, GLS, Sprinter. 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. Typical repair costs on Mercedes-Benz range from $300 to $3,500, depending on the specific model and root cause.
Common Causes of P20EE
- •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)
P20EE on Mercedes-Benz by Model
Mercedes-Benz GLE(1 issue)
- Diesel AdBlue / SCR NOx Sensor Fault Triggering Restart Countdown and No-Start2016-2024
On diesel GLEs, the SCR/AdBlue emissions system enforces compliance with a start countdown: when it detects a fault it warns 'You can restart the engine X more times,' and once the counter expires the car will not start until the fault is fixed and the counter is reset. The most common root cause is a failed upstream or downstream NOx sensor giving out-of-range readings; other causes are AdBlue crystallization, a blocked dosing valve, or low-quality fluid. Topping up AdBlue does NOT clear the countdown.
Mercedes-Benz GLS(1 issue)
- OM656 Diesel Emissions System Clogging (EGR/DPF/AdBlue SCR) and Limp Mode2020-2025
EU-market GLS 350d/400d models with the OM656 3.0L inline-six diesel suffer from sensitivity in the exhaust aftertreatment chain. The EGR (AGR) valve, diesel particulate filter (DPF) and SCR/AdBlue system are sensitive to poor fuel quality, short-trip driving and temperature swings. Clogged DPF, a dirty/sticking EGR valve, or a faulty EGR differential-pressure sensor cause power loss, failed regenerations, dashboard warnings and limp-home mode. A German motor-talk owner thread on the GLS X167 400d documents poor acceleration that resolves after a restart, traced to a replaced EGR differential-pressure sensor under warranty. This primarily affects diesels not sold in the US, so it is especially relevant to the EU market.
Mercedes-Benz Sprinter(1 issue)
- DEF/AdBlue SCR System Faults and Derate2014-2025
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.
Mercedes-Benz V-Class(1 issue)
- AdBlue/SCR System Faults Triggering Limp Mode and No-Start Countdown2015-2020
On Euro 6 W447 diesels the SCR emissions system depends on NOx sensors (before and after the catalyst) and the AdBlue dosing system. A failed NOx sensor, dosing fault, or low/contaminated AdBlue escalates from a yellow warning to a red warning and finally a 'No start in X miles' countdown, after which the engine will not restart once switched off. The vehicle commonly drops into limp mode (restricted speed/power) before that. NOx sensor failure is one of the most frequently cited W447 emissions faults.
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View P20EE across all makes →Frequently Asked Questions
What does P20EE mean on Mercedes-Benz?▼
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. On Mercedes-Benz specifically, this code is documented across 4 models.
What causes P20EE on Mercedes-Benz vehicles?▼
Common causes on Mercedes-Benz: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P20EE on a Mercedes-Benz?▼
Repair costs on Mercedes-Benz range from $300 to $3,500, depending on the specific model and root cause.
Which Mercedes-Benz models have P20EE documented?▼
Au7o has documented P20EE on 4 Mercedes-Benz models: GLE, GLS, Sprinter, V-Class.