SPN 3230 FMI 0AftertreatmentCriticalRegen Inhibit
Aftertreatment 1 SCR Catalyst Outlet Temperature — Above Normal (Most Severe)
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Connector Pinout — in guided session
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Overview
Fault Code
SPN 3230 / FMI 0
Category
Aftertreatment
Fault Type
sensor
FMI Category
above normal
Severity
Critical
Derate Risk
Yes
Regen Inhibit
Yes — active regen blocked
Inducement Risk
No
Component Location: SCR outlet temperature sensor at SCR catalyst outlet / tailpipe.
Common Symptoms
- SCR outlet temperature above maximum
- Thermal protection active — DEF dosing inhibited
- Engine derate active
- Possible SCR catalyst damage
Root Causes
- Uncontrolled regen propagating through SCR
- SCR outlet temperature sensor failure reading high
- Fuel doser leakage causing combustion in SCR
- SCR catalyst damage causing exothermic reaction
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Derate Behavior
Power Derate
Yes
Active Regen Inhibited
This fault prevents the DPF from completing an active regeneration cycle. DPF soot load will increase until the fault is resolved.
Mileage / Regen Notes
Random failure. More common with fuel doser issues.
System Interactions
SPN 3230 FMI 0 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: Temperature rise across SCR (outlet > inlet by >200°F) indicates catalyst damage.
Detroit
DD15: Inspect SCR catalyst for substrate damage after any over-temperature event.
PACCAR
MX-13: SCR catalyst replacement required if substrate is damaged.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3230 FMI 0 — 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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