SPN 3056 FMI 2AftertreatmentMedium
SCR Inlet NOx Sensor - Data Erratic
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DVOM Testing — in guided session
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Overview
Fault Code
SPN 3056 / FMI 2
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: SCR inlet NOx sensor is mounted in the exhaust system upstream of the SCR catalyst and DEF injection point.
Common Symptoms
- Erratic or jumping NOx readings on scan tool (values spike or drop unexpectedly), MIL illuminated, possible 5 MPH derate after extended time active, DEF dosing may become erratic or shut off, SCR efficiency codes may follow (SPN 4364), intermittent fault that clears and returns, poor fuel economy due to incorrect DEF dosing calculations
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
This fault typically does NOT inhibit active regeneration directly. However, if erratic NOx data causes the ACM to miscalculate SCR efficiency, the system may reduce or halt DEF dosing. Prolonged incorrect dosing can lead to elevated tailpipe NOx and trigger secondary SCR efficiency faults which CAN inhibit regen. Monitor SCR efficiency and DEF dosing rate alongside this code.
System Interactions
SPN 3056 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 universal TSBs — verify in OEM service portal. Cummins: Check for ISB/ISX NOx sensor harness routing bulletins. Detroit: DD13/DD15 may have harness chafe updates. Paccar: Check MX-13 NOx sensor connector service updates. Always search by engine serial number for applicable campaigns.
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 3056 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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