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Fault Code Reference

SPN 3057 FMI 2AftertreatmentMedium

SCR Outlet NOx Sensor - Data Erratic

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Overview

Fault Code

SPN 3057 / FMI 2

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: The SCR outlet NOx sensor is mounted in the exhaust system downstream of the SCR catalyst and DEF dosing module.

Common Symptoms

  • Erratic or jumping outlet NOx sensor readings on scan tool (values swing wildly or spike unexpectedly). Check Engine Light (CEL) or MIL illuminated. Possible SCR efficiency fault codes (SPN 4364) setting as secondary faults due to unreliable NOx data. Intermittent fault that may come and go with vibration or temperature changes. DEF dosing may appear erratic as the ACM struggles to interpret bad sensor data. Occasional white smoke if DEF dosing is affected. Driver may report fault clears after key cycle but returns.

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

This fault typically does NOT inhibit active DPF regeneration directly. However, if the erratic data causes secondary SCR efficiency faults (SPN 4364), those codes may inhibit regen. The ACM needs accurate outlet NOx data to calculate SCR conversion efficiency — erratic readings may cause the system to reduce DEF dosing as a protective measure, which can lead to elevated tailpipe NOx and eventual efficiency faults that do affect regen. Monitor for secondary fault codes.

System Interactions

SPN 3057 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
TSB Reference: No confirmed TSBs with specific bulletin numbers — verify in OEM service portal (Cummins QuickServe, Detroit DemandSide, PACCAR ESA, Navistar ISIS). Check for NOx sensor harness routing updates and connector service kits that have been released for various chassis applications.

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 3057 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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