SPN 143 FMI 3AftertreatmentInducement Risk
DEF Tank Level Sensor - Voltage Above Normal or Shorted High
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Overview
Fault Code
SPN 143 / FMI 3
Category
Aftertreatment
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
Yes — SCR inducement possible
Component Location: DEF tank level sensor is mounted directly on the DEF tank, typically located on the passenger-side frame rail or under cab, with wiring routed to the ACM.
Common Symptoms
- DEF tank level reading 100% constantly regardless of actual level
- DEF level gauge shows full at all times
- SCR system inducement warnings may appear
- Check engine light or aftertreatment malfunction indicator illuminated
- DEF low level warnings do not activate when tank is actually low
Root Causes
- DEF tank level sensor signal wire shorted to voltage (B+)
- Damaged or corroded DEF tank level sensor connector/wiring harness
- Failed DEF tank level sensor (internal short to supply voltage)
- Open or high resistance in sensor ground circuit causing voltage pull-up
- ACM sensor supply voltage fault feeding back through signal circuit
- Water or DEF contamination in the sensor connector causing leakage current
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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
An active SPN 143 FMI 3 fault may inhibit or disable DPF active regeneration due to inability to confirm adequate DEF supply level. SCR dosing may also be suspended, leading to potential NOx non-compliance and derate conditions depending on OEM fault management strategy.
System Interactions
SPN 143 FMI 3 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 143 FMI 3 — 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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