SPN 3031 FMI 0AftertreatmentCriticalRegen Inhibit
Aftertreatment 1 DPF Inlet Temperature - Data Valid But Above Normal (Most Severe)
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Overview
Fault Code
SPN 3031 / 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: DPF inlet temperature sensor threaded into the exhaust pipe immediately upstream of the DPF filter housing. Located in the aftertreatment assembly - typically under the cab or behind the cab on the frame rail.
Common Symptoms
- DPF inlet temperature reading critically high - above 650°C (1200°F)
- Active regeneration inhibited or terminated by ECM
- Engine derate active to reduce exhaust temperature
- Possible runaway regeneration - uncontrolled DPF temperature rise
- Exhaust temperature warning lamp illuminated
- Possible companion faults: SPN 3242 (DPF outlet temp high), SPN 3719 (soot load high)
Root Causes
- Runaway regeneration - excessive soot load igniting uncontrolled in DPF
- Fuel in exhaust - injector leak, post-injection fault, or in-cylinder fuel delivery issue
- DPF inlet temperature sensor failure - reading falsely high
- Exhaust brake or retarder generating excessive heat during regen
- Incorrect regen inhibit switch - regen occurring under conditions not suitable for high temp
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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
DPF temperature sensor failure increases with mileage and heat cycling. Runaway regen risk increases with DPF soot load - more common in vehicles with frequent regen inhibit use.
System Interactions
SPN 3031 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 4021834 - DPF Temperature Sensor Diagnosis and Runaway Regen Procedure; Detroit TSB 18-005 - DPF Inlet Temperature Above Normal 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: Runaway regen is a known risk on high-soot-load DPFs. INSITE DPF regen test required. If runaway regen suspected, do NOT perform active regen - allow DPF to cool before diagnosis.
Detroit
DD15: Fuel in exhaust from injector tip wear is a common cause of DPF overtemperature. DDDL injector test required alongside DPF diagnosis.
PACCAR
MX-13: DPF inlet temp sensor is exposed to high heat cycling - sensor failure is common at 300k+ miles. Replace sensor if resistance is out of spec.
Volvo
MP8: Check exhaust brake operation - excessive exhaust brake use during regen can cause DPF overtemperature. PTT exhaust brake test required.
Related Faults in This Category
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3031 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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