What are the symptoms of SPN 4364 FMI 18?

SCR efficiency warning lamp illuminated. Engine derate active or imminent - typically 25–50% power reduction. NOx emissions above threshold - may fail roadside emissions check. DEF consumption appears normal but NOx not reducing. Possible companion faults: SPN 3226 (downstream NOx), SPN 3480 (SCR efficiency critical). Inducement countdown may be active if fault has been present for extended miles

What causes SPN 4364 FMI 18?

SCR catalyst degraded or poisoned - sulfur, oil, or coolant contamination. DEF dosing rate incorrect - doser clogged, worn, or out of calibration. DEF quality out of specification - diluted urea reduces NOx conversion. Upstream NOx sensor failure - false high reading makes efficiency appear low. Downstream NOx sensor failure - false high reading makes efficiency appear low. SCR catalyst temperature too low - catalyst not at light-off temperature (>200°C)

How do I diagnose SPN 4364 FMI 18?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 4364 FMI 18 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 4364 FMI 18AftertreatmentHighInducement Risk

Aftertreatment 1 SCR Conversion Efficiency - Data Valid But Below Normal (Moderately Severe)

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Overview

Fault Code

SPN 4364 / FMI 18

Common Symptoms

SCR efficiency warning lamp illuminated
Engine derate active or imminent - typically 25–50% power reduction
NOx emissions above threshold - may fail roadside emissions check
DEF consumption appears normal but NOx not reducing
Possible companion faults: SPN 3226 (downstream NOx), SPN 3480 (SCR efficiency critical)
Inducement countdown may be active if fault has been present for extended miles

Root Causes

SCR catalyst degraded or poisoned - sulfur, oil, or coolant contamination
DEF dosing rate incorrect - doser clogged, worn, or out of calibration
DEF quality out of specification - diluted urea reduces NOx conversion
Upstream NOx sensor failure - false high reading makes efficiency appear low
Downstream NOx sensor failure - false high reading makes efficiency appear low
SCR catalyst temperature too low - catalyst not at light-off temperature (>200°C)

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

AI walks you through every test — one step at a time. Plans start at $12.95/mo.

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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

SCR catalyst efficiency degrades gradually with mileage. Significant degradation typically occurs at 400k–600k miles. DEF quality issues can cause premature degradation at any mileage.

System Interactions

SPN 4364 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

TSB Reference: Cummins TSB 4021889 - SCR Efficiency Diagnosis; Detroit TSB 18-012 - SCR Conversion Efficiency Below Normal; PACCAR TSB AT-002 - MX-13 SCR Efficiency Diagnosis

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.

OEM Notes

Cummins

X15: SCR catalyst replacement is common at 500k+ miles. Before condemning catalyst, verify DEF quality and doser operation. INSITE SCR efficiency test required.

Detroit

DD15: Sulfur poisoning from low-quality diesel accelerates SCR catalyst degradation. DDDL aftertreatment monitor shows real-time efficiency. Catalyst desulfation procedure available in DDDL.

PACCAR

MX-13: DEF doser clogging is the most common cause on MX-13. Inspect doser tip before condemning catalyst. DAVIE4 doser flow test required.

Volvo

MP8: Downstream NOx sensor failure is common on MP8 - sensor reads high falsely, making efficiency appear low. Replace downstream sensor first if no other faults present.