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Fault Code Reference

SPN 4787 FMI 31AftertreatmentInducement Risk

Aftertreatment SCR Operator Inducement Severity - Condition Exists

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Overview

Fault Code

SPN 4787 / FMI 31

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: ACM (Aftertreatment Control Module) governs SCR inducement logic; SCR catalyst and DEF system are located in the exhaust aftertreatment assembly under or behind the cab.

Common Symptoms

  • DEF quality warning lamp illuminated
  • Aftertreatment malfunction indicator lamp (MIL) active
  • Driver display shows SCR system warning message
  • Vehicle speed or torque limiting active
  • Reduced engine performance
  • DEF consumption abnormal or zero
  • Possible check engine light (amber)
  • Driver notification of severe derate countdown

Root Causes

  • DEF quality out of specification (diluted, contaminated, or wrong fluid in DEF tank)
  • DEF tank level critically low or empty
  • SCR NOx conversion efficiency below threshold triggering inducement
  • Faulty DEF quality sensor providing incorrect concentration readings
  • Expired or degraded DEF (urea concentration outside 31.8–33.2% range)
  • SCR catalyst degraded or poisoned reducing NOx conversion
  • NOx sensor (upstream or downstream) malfunction causing false efficiency calculation
  • DEF dosing system fault causing insufficient reagent delivery to SCR
  • ACM software or calibration issue incorrectly triggering inducement logic
  • Prior unresolved SCR-related fault codes accumulating inducement severity

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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 4787 FMI 31 indicates an operator inducement condition is active. DPF regeneration may be inhibited or limited as part of the inducement ladder. As inducement severity increases, engine power derate and speed limiting will occur, and regen events will be suppressed until the root SCR non-compliance condition is resolved and the inducement counter is reset.

System Interactions

SPN 4787 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 4787 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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