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Fault Code Reference

SPN 3361 FMI 31AftertreatmentInducement Risk

Aftertreatment 1 DEF Dosing Air Assist Absolute Pressure - Condition Exists

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Overview

Fault Code

SPN 3361 / FMI 31

Category

Aftertreatment

Derate Risk

Possible

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: The DEF dosing air assist pressure sensor is located on the DEF dosing unit module mounted near the exhaust aftertreatment system.

Common Symptoms

  • Poor DEF atomization and mixing
  • DEF crystallization in decomposition tube
  • Increased NOx emissions
  • Poor SCR conversion efficiency
  • SCR system underperformance
  • DEF quality deterioration fault codes
  • Reduced engine power in some cases

Root Causes

  • DEF dosing air supply pressure out of specification (too low or too high) Clogged or restricted air supply line to DEF dosing unit Faulty or stuck DEF air pressure regulator Leaking air supply fitting or cracked air line at dosing module Failed or degraded DEF dosing air pressure sensor Air compressor not supplying adequate pressure to aftertreatment system Contaminated or frozen DEF air assist lines Faulty ACM (Aftertreatment Control Module) misreading sensor signal

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Derate Behavior

Power Derate

Possible

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 3361 FMI 31 can inhibit active DPF regeneration because the DEF/reductant dosing air assist system must be operational for proper SCR and DPF system function. If air assist pressure is out of range, the ACM may suspend dosing and block regen events to prevent SCR catalyst damage.

System Interactions

SPN 3361 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 3361 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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