Fault Family System: Powertrain (P-Codes) 9 SAE Codes

Reductant Control Module Supply

Diagnostic & Technical Explanation

P20EA (Reductant Control Module Power Relay De-Energized Performance - Too Early) is logged by the reductant/DEF control module itself when its own power relay circuit, battery supply voltage, or internal processing is out of specification, rather than a fault in any downstream sensor or actuator. A fatigued main power relay, a corroded ground strap at the module, and a battery/charging-system voltage sag under load are the most common root causes, and this module often stays powered after key-off to finish a heater or purge cycle, so a parasitic-draw complaint can share the same root cause.

These 9 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 9 times.

Codes in this family (9)

Code	Definition	Repair coverage
P20EA	Reductant Control Module Power Relay De-Energized Performance - Too Early	4 ranked fixes
P20EB	Reductant Control Module Power Relay De-Energized Performance - Too Late	4 ranked fixes
P20FF	Reductant Control Module Performance	4 ranked fixes
P21C7	Reductant Control Module Power Relay/Relays Control Circuit/Open	4 ranked fixes
P21C8	Reductant Control Module Power Relay/Relays Control Circuit Low	4 ranked fixes
P21C9	Reductant Control Module Power Relay/Relays Control Circuit High	4 ranked fixes
P21CA	Reductant Control Module Supply Voltage Circuit	4 ranked fixes
P21CB	Reductant Control Module Supply Voltage Low	4 ranked fixes
P21CC	Reductant Control Module Supply Voltage High	4 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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