Fault Family System: Powertrain (P-Codes) 5 SAE Codes

SCR NOx Catalyst Efficiency

Diagnostic & Technical Explanation

P20EE (SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1) sets when the ECM's NOx conversion model shows the SCR catalyst (or its upstream pre-catalyst stage) is not reducing NOx as effectively as expected for the reductant dosed. Catalyst washcoat aging or thermal damage, a NOx sensor pair reading a smaller-than-expected drop across the catalyst, and chronic under-dosing from a partially clogged injector are the leading causes, in that order of likelihood.

These 5 diagnostic trouble codes share one detection mechanism and one repair path, which is why MechanicDB groups them into a single authored fault family rather than repeating the same explanation 5 times.

Codes in this family (5)

Code	Definition	Repair coverage
P20EE	SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 1	3 ranked fixes
P20EF	SCR NOx Pre- Catalyst Efficiency Below Threshold Bank 1	3 ranked fixes
P20F2	SCR NOx Catalyst Efficiency Below Threshold Bank 2	3 ranked fixes
P20F3	SCR NOx Pre- Catalyst Efficiency Below Threshold Bank 2	3 ranked fixes
P2C7A	SCR NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 2	3 ranked fixes

What this page does not show

This page publishes the fault mechanism and the code index. The licensed dataset adds, for every one of the 15,886 codes, the full authored repair set (56,729 procedures in total) with the fields deliberately withheld here:

Procedures 2..n after the first-listed procedure
Aftermarket parts cost ranges in USD (est_parts_cost_min/max_usd)
Professional labor-hour estimates (est_labor_hours)
Step-by-step diagnostic and repair instructions
Replacement-part mappings and catalog lookups
6,637 manufacturer-specific OEM codes across 32 makes ($149 tier)

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