SPN 3363 FMI 2AftertreatmentInducement Risk
Aftertreatment Diesel Exhaust Fluid Quality - Data Erratic
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Overview
Fault Code
SPN 3363 / FMI 2
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: DEF quality sensor is located on or near the DEF tank, typically mounted in-tank or at the tank outlet on the chassis frame rail beneath the cab.
Common Symptoms
- DEF quality warning light illuminated
- Engine derate or power reduction
- DEF system fault messages on dash
- Increased exhaust emissions
- SCR system efficiency reduced message
- Check engine light active
Root Causes
- Contaminated DEF (diluted with water, urea concentration out of 32.5% specification range) Wrong fluid in DEF tank (diesel, water, or other fluid instead of DEF) Failed or degraded DEF quality sensor (reductant quality sensor) DEF tank contamination from previous incorrect fill DEF quality sensor connector corrosion or water intrusion Frozen or partially frozen DEF affecting sensor reading DEF quality sensor wiring harness damage or chafing Low-quality or off-brand DEF not meeting ISO 22241 specification ACM software or calibration issue causing erratic data interpretation
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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 3363 FMI 2 (DEF quality erratic) will typically inhibit or limit SCR dosing, which can lead to SCR system deactivation. DPF passive and active regeneration may continue, but NOx aftertreatment efficiency is reduced. Depending on ACM logic and fault severity, the system may enter a derate condition with regeneration inhibit if the fault persists across multiple drive cycles. Operator inducement (torque derate or idle-only) may be enforced after a defined distance or engine hours threshold is exceeded.
System Interactions
SPN 3363 FMI 2 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3363 FMI 2 — 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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