Your 2021 Mercedes-Benz GLE diesel has displayed an AdBlue warning, and a scan reveals P20EE—SCR NOx Catalyst Efficiency Below Threshold. This emissions-related code indicates the selective catalytic reduction system isn't performing properly, requiring attention before the vehicle potentially restricts operation.
Understanding the AdBlue/SCR System
Modern Mercedes diesel engines use Selective Catalytic Reduction (SCR) to meet stringent emissions standards. The system injects a urea solution (AdBlue/DEF) into the exhaust stream, where it converts harmful nitrogen oxides (NOx) into harmless nitrogen and water in the SCR catalyst. When P20EE appears, the system isn't achieving expected NOx reduction levels.
What P20EE Means
This code indicates the SCR catalyst efficiency has fallen below the threshold the engine computer expects. The system monitors NOx levels before and after the SCR catalyst—if the reduction percentage is too low, P20EE sets. This could indicate problems with:
- AdBlue quality or contamination - Diluted or contaminated fluid
- AdBlue delivery system - Injector, pump, or line problems
- SCR catalyst degradation - The catalyst itself has lost effectiveness
- NOx sensor malfunction - Faulty sensors giving incorrect readings
- Exhaust temperature issues - SCR requires specific temperatures to function
Common Causes in the GLE Diesel
The diesel GLE (GLE 350d or similar) typically experiences P20EE from:
- Crystallized AdBlue in injector - Common after short-trip driving
- NOx sensor failure - These sensors are known wear items
- AdBlue pump failure - Unable to deliver proper fluid quantity
- SCR catalyst contamination - Engine problems affecting the catalyst
Warning Escalation
Mercedes diesel vehicles with SCR faults will progressively limit operation:
- Initial warning with remaining start count
- Countdown of remaining engine starts
- Speed limitation
- No-start condition (in some regions)
Repair Costs for 2021 GLE Diesel
- AdBlue system flush and refill: $200-$400
- AdBlue injector replacement: $400-$800
- NOx sensor replacement: $500-$900
- AdBlue pump replacement: $600-$1,200
- SCR catalyst replacement: $2,500-$5,000