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P20EE SCR NOx catalyst efficiency below threshold, bank 1

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SCR NOx catalyst efficiency below threshold, bank 1, readable through chat.

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System
Emissions control
Component
SCR catalyst

Symptoms

  • Check engine light on
  • Reduced power or speed limit if the fault is left unresolved

Common causes

  • PCM software out of date
  • Degraded, diluted or incorrect DEF
  • DEF leak in the dosing system
  • Exhaust system leak
  • DEF deposits in the decomposition tube
  • Malfunctioning DEF injector, supply line or supply pump
  • Defective SCR catalyst

Guides for this code

Pencil sketch of a Ram 2500 in a garage
Diagnostics & Repair

P20EE on a 2017-2018 Ram 2500/3500 Cummins: SCR Fault, Causes, Fix

JC Landry · Oct 27, 2026

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