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Fault Code Reference

SPN 3702 FMI 18AftertreatmentInducement Risk

NOx Conversion Efficiency - Data Valid But Below Normal

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Overview

Fault Code

SPN 3702 / FMI 18

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: SCR catalyst assembly is mounted in the aftertreatment housing on the frame rail or beneath cab, downstream of the DPF, with NOx sensors at SCR inlet and outlet.

Common Symptoms

  • Check engine light illuminated
  • Engine derate or power reduction
  • DEF consumption may increase
  • Elevated NOx emissions detected
  • Aftertreatment system warning messages

Root Causes

  • Degraded or poisoned SCR catalyst (sulfur, oil, or coolant contamination) DEF quality issue — diluted, contaminated, or incorrect concentration (not 32.5% urea) DEF dosing system malfunction — clogged or worn DEF injector reducing urea delivery DEF supply pump delivering insufficient pressure or flow SCR catalyst aging or thermal damage from repeated high-temperature events NOx sensor (downstream) reading inaccurately — skewing efficiency calculation NOx sensor (upstream) reading inaccurately — false low inlet NOx reading inflating apparent efficiency DEF line restriction or frozen DEF in cold-weather operation DPF excessive soot loading upstream causing abnormal exhaust temperature profile through SCR Engine-out NOx elevated due to engine fault (EGR malfunction, fueling issue) — overwhelming SCR Incorrect ACM calibration or software version mismatched to hardware

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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 3702 FMI 18 indicates low NOx conversion efficiency and will inhibit active DPF regeneration once fault becomes active-inducing, as the ACM requires a functioning SCR system before initiating regen. Continued low efficiency faults may trigger a derate and eventually force a parked or service regen requirement. If SCR efficiency remains below threshold, the system may enter a locked-out state requiring dealer-level INSITE intervention to clear.

System Interactions

SPN 3702 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 3702 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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