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Fault Code Reference

SPN 5357 FMI 31AftertreatmentInducement Risk

Aftertreatment DEF Dosing Malfunction - Condition Exists

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Overview

Fault Code

SPN 5357 / FMI 31

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF dosing system components are located along the exhaust aftertreatment assembly, with the DEF doser injector mounted on the decomposition tube upstream of the SCR catalyst, typically on the driver side of the chassis.

Common Symptoms

  • DEF consumption higher or lower than normal
  • Poor NOx conversion efficiency
  • Amber MIL or aftertreatment warning lamp illuminated
  • NOx sensor readings out of expected range
  • SCR efficiency below threshold
  • Potential white smoke from exhaust

Root Causes

  • DEF quality out of specification (contaminated or diluted DEF fluid) DEF doser injector clogged or stuck closed DEF supply module (pump) failure or low output pressure DEF lines frozen or kinked restricting flow DEF tank level sensor fault causing incorrect readings DEF doser injector electrical fault or harness damage ACM software or calibration issue requiring reflash DEF filter clogged restricting flow to doser DEF tank heater malfunction causing freezing in cold climates Air purge system failure preventing proper doser operation

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Derate Behavior

Power Derate

Yes

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

Mileage / Regen Notes

SPN 5357 FMI 31 indicates an active DEF dosing malfunction which will inhibit or severely restrict SCR system operation. DPF passive and active regeneration may be inhibited due to compromised aftertreatment system integrity. The engine may enter a derate condition with restricted vehicle speed or power if the fault persists without correction.

System Interactions

SPN 5357 FMI 31 interacts with the following systems and may trigger cascading faults if not resolved promptly.

DPF differential pressure monitoring — soot load calculations affected
SCR NOx conversion efficiency — DEF dosing strategy may be altered
Active regen inhibit logic — regen may be blocked or delayed
Engine protection derate — ECM may reduce power to protect components

Common Diagnostic Mistakes

Replacing the sensor without verifying wiring continuity and supply voltage first.

Clearing codes without identifying and resolving the root cause — the fault will return.

Ignoring related active faults — cascading faults often share a root cause.

Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.

Condemning the ECM before exhausting all wiring and component tests.

Replacing the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.

Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.

Deep-dive diagnostic page: SPN 5357 FMI 31 — Full FSM Sequence & Repair Steps — includes wiring notes, sensor specs, and gated repair procedures.

Authority References

Diagnostic procedures on this page are informed by OEM service literature and industry standards.

EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.

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