SPN 3216 FMI 20AftertreatmentMedium
Aftertreatment SCR Outlet NOx Sensor — Data Drifted High
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3216 / FMI 20
Category
Aftertreatment
Fault Type
sensor
FMI Category
data erratic
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: SCR outlet — downstream of SCR catalyst in exhaust stream
Common Symptoms
- NOx efficiency fault active
- Outlet NOx reading higher than expected
- Possible false SCR efficiency fault
Root Causes
- NOx sensor contamination or aging
- NOx sensor heater circuit fault
- Exhaust leak downstream of SCR affecting sensor reading
- Sensor calibration drift
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
Sensor drift common after 300k miles. Replace proactively at 400k–500k.
System Interactions
SPN 3216 FMI 20 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: Cummins TSB 4021514 — NOx Sensor Replacement Procedure
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.
OEM Notes
Cummins
ISX15: Replace NOx sensor as assembly. Do not attempt to clean.
Detroit
DD15: DDDL NOx Sensor Rationality Test available.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3216 FMI 20 — 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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