SPN 3250 FMI 4AftertreatmentMedium
DOC Outlet Temperature Sensor - Voltage Low
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 3250 / FMI 4
Category
Aftertreatment
Severity
Medium
Derate Risk
Sometimes
Regen Inhibit
No
Inducement Risk
No
Component Location: DOC outlet temperature sensor is mounted on the aftertreatment assembly downstream of the DOC brick, typically on the DPF inlet canister or connecting pipe under the vehicle chassis.
Common Symptoms
- Check engine light illuminated. Amber warning lamp on dash. DOC outlet temperature reading abnormally low or showing -40°F/-40°C on scan tool. Possible white or black smoke during cold starts due to improper aftertreatment temperature management. May notice reduced fuel economy if system compensates with extra fuel injection for regen attempts. Diagnostic scan shows DOC outlet temp stuck at minimum value or zero.
Root Causes
- [object Object]
- [object Object]
- [object Object]
- [object Object]
- [object Object]
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
Sometimes
Mileage / Regen Notes
This fault typically inhibits active regeneration. The ACM relies on DOC outlet temperature to verify catalyst light-off and control hydrocarbon dosing. With voltage low (reading invalid), the system cannot confirm DOC is at proper temperature for safe regen. Parked regen may also be blocked depending on OEM strategy. Soot loading will continue to increase until fault is resolved. On some systems, regen may attempt but abort early due to invalid temperature feedback.
System Interactions
SPN 3250 FMI 4 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 universal TSBs — verify in OEM service portal. Cummins, Detroit, and Paccar have issued service letters on aftertreatment sensor harness routing and heat shield interference. Check for chassis-specific TSBs related to sensor harness chafing near DPF/DOC assembly.
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 3250 FMI 4 — 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 3250 FMI 4
The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.