SPN 3226 FMI 16AftertreatmentHigh
Aftertreatment 1 SCR Catalyst Inlet Temperature — Above Normal (Moderately Severe)
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
DieselVision™ Upload photos — DieselVision™ reads diagrams, connectors, and screens with precision.
Overview
Fault Code
SPN 3226 / FMI 16
Category
Aftertreatment
Fault Type
sensor
FMI Category
above normal
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: SCR inlet temperature sensor at SCR catalyst inlet.
Common Symptoms
- SCR inlet temperature above warning threshold
- Warning lamp active
- DEF dosing may be reduced
- Temperature elevated but not at maximum
Root Causes
- Extended regen cycle elevating exhaust temperature
- High ambient temperature and load conditions
- SCR inlet temperature sensor reading slightly high
- High soot loading requiring extended regen
Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.
AI walks you through every test — one step at a time. Plans start at $12.95/mo.
Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Higher frequency with high soot loading.
System Interactions
SPN 3226 FMI 16 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 4021901 — Aftertreatment Temperature Sensor Diagnosis
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
X15: FMI 16 is a warning level. Monitor closely.
Detroit
DD15: Check regen frequency — excessive regen indicates upstream issue.
PACCAR
MX-13: High soot loading from EGR issues is primary cause.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3226 FMI 16 — 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.
Get weekly diesel diagnostic tips
Real-world fault code walkthroughs, wiring tips, and shop-tested fixes — free.
No spam. Unsubscribe anytime.
Get AI-Guided Diagnosis for SPN 3226 FMI 16
The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.