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Fault Code Reference

SPN 431 FMI 0EmissionsMedium

Warm Up Catalyst Efficiency Below Threshold (Bank 2)

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Overview

Fault Code

SPN 431 / FMI 0

Category

Emissions

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: Warm-up catalyst (close-coupled catalytic converter) is mounted directly on or near the Bank 2 exhaust manifold, forward of the main underfloor catalyst.

Common Symptoms

  • MIL (Check Engine Light) illuminated
  • Failed emissions inspection or OBD readiness monitor not complete
  • Possible slight increase in exhaust smell during cold start
  • DTC stored in ECM with freeze frame data at cold start conditions
  • Possible reduced fuel economy noted by driver

Root Causes

  • Degraded or aged warm-up catalytic converter substrate (Bank 2) below efficiency threshold
  • Exhaust leaks upstream of Bank 2 warm-up catalyst skewing sensor readings
  • Failed or lazy downstream oxygen sensor providing inaccurate efficiency comparison
  • Failed or contaminated upstream O2 sensor on Bank 2
  • Engine oil or coolant intrusion causing catalyst poisoning

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Derate Behavior

Mileage / Regen Notes

SPN 431 FMI 0 is a gasoline/light-duty OBD-II emissions code and does not directly inhibit diesel DPF regeneration cycles; this code is not applicable to heavy-duty diesel DPF regen logic.

System Interactions

SPN 431 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.

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.

Deep-dive diagnostic page: SPN 431 FMI 0 — 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.

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.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.

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