SPN 3250 FMI 2Medium
DOC Outlet Temperature - Data Erratic
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 3250 / FMI 2
Severity
Medium
Regen Inhibit
No
Inducement Risk
No
Component Location: DOC outlet temperature sensor is mounted on the exhaust pipe immediately downstream of the Diesel Oxidation Catalyst, typically mid-chassis under the cab or frame rail.
Common Symptoms
- Active fault code SPN 3250 FMI 2 stored in aftertreatment module
- Erratic or fluctuating DOC outlet temperature readings on diagnostic tool
- DPF regeneration inhibited or interrupted mid-cycle
- MIL or amber warning lamp illuminated
- Inconsistent exhaust temperature values compared to DOC inlet sensor
- Engine derate may be active during extended fault duration
Root Causes
- Faulty or degraded DOC outlet temperature sensor (thermistor failure causing signal drift)
- Loose, corroded, or damaged connector at DOC outlet temperature sensor
- Chafed or shorted wiring harness between sensor and aftertreatment control module
- Moisture intrusion into sensor connector causing intermittent signal
- Sensor contamination from soot or oil fouling affecting response
- Failed aftertreatment control module (rare)
- Exhaust system vibration causing intermittent sensor contact
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Derate Behavior
Mileage / Regen Notes
An erratic DOC outlet temperature signal will inhibit or abort active DPF regeneration cycles, as the ACM relies on this sensor to confirm DOC light-off and manage regen temperature thresholds. Forced or parked regens may not initiate while the fault is active.
System Interactions
SPN 3250 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: DOC temp TSB
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.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 3250 FMI 2 — 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.
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.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.
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