SPN 3057 FMI 18Aftertreatment
NOx Sensor 2 Preliminary FMI - Data Valid But Below Normal
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Overview
Fault Code
SPN 3057 / FMI 18
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: NOx Sensor 2 is located in the exhaust outlet pipe downstream of the SCR catalyst, typically within 12–24 inches of the SCR catalyst exit on the aftertreatment assembly.
Common Symptoms
- Check engine light illuminated
- NOx sensor implausibility fault stored
- Aftertreatment system efficiency warnings
- Possible engine derate depending on OEM calibration
- DEF consumption rate abnormal
- SCR performance degradation messages
Root Causes
- Faulty or degraded downstream (post-SCR) NOx sensor 2 reporting low NOx values NOx sensor 2 contaminated by water intrusion or exhaust condensate Defective NOx sensor 2 heater element causing slow warm-up and incorrect readings Wiring harness chafing or intermittent open circuit on NOx sensor 2 signal circuit Corroded or damaged NOx sensor 2 connector causing signal degradation SCR catalyst over-converting (unusually efficient) masking true NOx output — rare DEF dosing system over-injecting urea leading to artificially suppressed downstream NOx ECM/ACM calibration or software issue misinterpreting NOx sensor 2 signal NOx sensor 2 CAN communication error causing invalid low value reporting
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
FMI 18 on NOx Sensor 2 may inhibit or delay active DPF regeneration if the ACM determines SCR/DEF system performance cannot be confirmed. The system may also trigger a NOx conversion efficiency derate or SCR inducement timer if the fault persists across multiple drive cycles. Forced or parked regens may be blocked until the NOx sensor fault is resolved on some platforms.
System Interactions
SPN 3057 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3057 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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