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Fault Code Reference

SPN 4331 FMI 1Aftertreatment

Aftertreatment 1 Outlet Particulate Matter Sensor - Data Valid But Below Normal

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Overview

Fault Code

SPN 4331 / FMI 1

Category

Aftertreatment

Derate Risk

No

Regen Inhibit

No

Inducement Risk

No

Component Location: Outlet PM sensor is mounted in the exhaust bung on the downstream (outlet) end of the DPF canister, typically on the mid or rear undercarriage aftertreatment assembly.

Common Symptoms

  • Active DTC for SPN 4331 FMI 1 stored in ACM
  • PM sensor reading abnormally low or near zero
  • DPF regeneration cycle frequency may increase or become erratic
  • MIL or amber warning lamp illuminated
  • Possible inhibited or forced regen conditions
  • Reduced aftertreatment system confidence in soot load estimation

Root Causes

  • PM sensor contaminated with ash, oil fouling, or excessive soot plugging the sensing element PM sensor element failure or degraded sensing electrode Exhaust leak upstream of outlet PM sensor causing diluted exhaust sample Sensor regeneration cycle (burn-off) not completing properly due to heater circuit fault Wiring harness damage, chafing, or short to ground on signal circuit Poor or corroded connector at PM sensor harness connector Incorrect sensor installation or wrong part number installed ACM software or calibration issue requiring reflash

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Derate Behavior

Power Derate

No

Mileage / Regen Notes

SPN 4331 FMI 1 can inhibit or alter DPF regeneration scheduling. Because the ACM relies on the outlet PM sensor to confirm DPF efficiency and soot breakthrough, a below-normal reading may cause erratic regen initiation, inhibit active regen, or trigger a forced regen. Some platforms will inhibit parked regen until the fault is resolved.

System Interactions

SPN 4331 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 4331 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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