SPN 3556 FMI 3AftertreatmentMedium
Regeneration Temperature Sensor - Voltage High
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3556 / FMI 3
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: Regeneration temperature sensor is typically mounted on the DPF inlet or outlet housing in the aftertreatment assembly, located under the cab or along the frame rail.
Common Symptoms
- Check engine light illuminated; MIL lamp on; DPF regeneration may not complete properly; possible white or black smoke during attempted regen; scan tool shows regeneration temperature reading pegged at maximum value or showing implausible high reading; may see related aftertreatment efficiency codes; truck may go into reduced power mode after extended operation without successful regen
Root Causes
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Derate Behavior
Power Derate
Sometimes
Mileage / Regen Notes
This fault typically does NOT inhibit active regeneration outright, but the ECM/ACM may limit regen duration or effectiveness due to unreliable temperature feedback. System cannot accurately monitor DPF outlet or inlet temperature, which may cause regen to abort early or run incomplete cycles. Over time, this can lead to excessive soot loading and eventual DPF-related derates. If fault is active during regen attempt, monitor for related soot level and efficiency codes.
System Interactions
SPN 3556 FMI 3 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 — verify in OEM service portal. Check for manufacturer-specific bulletins related to aftertreatment harness routing, connector sealing updates, and regen temperature sensor revisions. Cummins ISX and ISB platforms have had harness chafe issues near DPF; Detroit DD13/DD15 have documented connector corrosion concerns at aftertreatment sensors.
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 3556 FMI 3 — 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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