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Fault Code Reference

SPN 3610 FMI 18Aftertreatment

Aftertreatment 1 Diesel Oxidation Catalyst Conversion Efficiency

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Overview

Fault Code

SPN 3610 / FMI 18

Category

Aftertreatment

Derate Risk

Possible

Regen Inhibit

No

Inducement Risk

No

Component Location: The DOC is located in the aftertreatment assembly mounted under the vehicle frame or on the chassis rail, typically ahead of the DPF canister in the exhaust flow path.

Common Symptoms

  • Check engine light illuminated
  • Poor DPF regeneration performance
  • Increased soot loading rates
  • Elevated exhaust backpressure
  • Reduced fuel economy
  • Increased DPF regeneration frequency

Root Causes

  • Aged or degraded DOC substrate with reduced precious metal catalyst activity
  • DOC contaminated by excessive oil consumption (ash/phosphorus poisoning)
  • DOC contaminated by coolant intrusion (silicate poisoning)
  • Insufficient exhaust temperatures for DOC light-off due to low load duty cycles
  • Failed or lazy exhaust temperature sensor upstream or downstream of DOC skewing efficiency calculation
  • Excessive sulfur content in fuel causing temporary DOC sulfur poisoning
  • DPF regeneration events not reaching sufficient temperatures to desorb sulfur from DOC
  • Cracked or cored-out DOC substrate reducing catalytic surface area
  • Exhaust leaks upstream of DOC reducing measured temperature differential

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Derate Behavior

Power Derate

Possible

Mileage / Regen Notes

SPN 3610 FMI 18 indicates DOC conversion efficiency is below expected threshold. Active and parked DPF regenerations may be inhibited or degraded because the DOC cannot sufficiently oxidize HC to generate the exotherm required to elevate DPF temperatures for soot combustion. This can result in increasing DPF soot load, more frequent regen requests, incomplete regens, and potential DPF damage if condition persists.

System Interactions

SPN 3610 FMI 18 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 3610 FMI 18 — 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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