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Fault Code Reference

SPN 3359 FMI 1AftertreatmentInducement Risk

Aftertreatment 1 Diesel Exhaust Fluid Dosing Unit Input Lines - Data Valid But Below Normal

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Overview

Fault Code

SPN 3359 / FMI 1

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: DEF dosing unit and supply input lines are located along the exhaust system downstream of the DPF, typically mid-chassis or frame-rail routed from the DEF tank to the SCR catalyst dosing point.

Common Symptoms

  • DEF low dosing or insufficient DEF injection warning
  • Engine derate or power reduction
  • SCR system efficiency fault codes present
  • Check engine light or amber warning lamp illuminated
  • DEF consumption lower than expected
  • NOx conversion efficiency reduced

Root Causes

  • Low DEF level in tank causing air ingestion or starvation at dosing unit Clogged or restricted DEF supply line (crystallized urea blockage) Failed or weak DEF supply pump delivering insufficient pressure/flow to dosing unit Frozen DEF supply line or dosing unit (cold weather operation) DEF injector (dosing unit) inlet screen plugged with DEF crystals Faulty DEF pressure sensor reporting low pressure erroneously Air leak in DEF supply line causing low pressure at dosing unit inlet DEF quality issue (diluted or contaminated DEF) causing system anomaly Failed DEF heating element not thawing frozen lines in cold conditions ACM software anomaly or calibration issue misinterpreting line pressure data

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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 3359 FMI 1 (DEF dosing unit input lines below normal) will inhibit active DPF regeneration and SCR dosing. The ACM will suspend DEF injection to prevent dry-firing the dosing unit. Extended fault condition will trigger SCR derate and potential engine torque/speed derate (inducement) per EPA guidelines if NOx aftertreatment is non-functional.

System Interactions

SPN 3359 FMI 1 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 3359 FMI 1 — 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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