Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairHard
A diesel vehicle equipped with a Selective Catalytic Reduction (SCR) system is brought in with a complaint of reduced engine power and a 'check engine' light illuminated. The scan tool reveals a P20EE (SCR NOx Catalyst Efficiency Below Threshold Bank 1) DTC. The Diesel Exhaust Fluid (DEF) level is full, and the DEF quality sensor shows normal readings. Which of the following is the MOST likely cause?
- AA clogged DPF.
- BA malfunctioning DEF injector (doser).
- CA faulty DPF differential pressure sensor.
- DA failing NOx sensor.
Show answer & explanationAnswer & explanation
Correct answer: B. A malfunctioning DEF injector (doser).
The P20EE code indicates that the SCR system is not effectively reducing NOx emissions. Since the DEF level and quality are confirmed good, the most direct cause for a lack of NOx reduction would be the DEF injector (doser) not spraying the correct amount of DEF into the exhaust stream, preventing the SCR catalyst from performing its function.
Why the other options are wrong
- A. A clogged DPF would cause DPF-specific codes (e.g., P2002, P2459) and high exhaust back pressure, but it doesn't directly cause a P20EE code without impacting DEF dosing.
- C. A faulty DPF differential pressure sensor would typically cause DPF-related codes (e.g., P2453) and might affect regeneration, but not directly P20EE.
- D. While a NOx sensor is critical for monitoring SCR efficiency, a failing sensor would typically generate codes like P2200 (NOx Sensor Circuit Malfunction) or P2201 (NOx Sensor Performance), not directly P20EE unless it's falsely reporting low efficiency.
SCR System Dosing Malfunction
Issues with the Diesel Exhaust Fluid (DEF) injector (doser) preventing proper introduction of urea into the exhaust stream, leading to inefficient NOx reduction and related DTCs.
- P20EE indicates poor SCR catalyst efficiency.
- DEF injector is crucial for NOx conversion.
- Clogging is a common failure mode for DEF injectors.
Memory trick: P20EE, DEF's free, but the doser's not working for thee!