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Fault Code Reference

SPN 3250 FMI 3AftertreatmentMedium

DOC Outlet Temperature Sensor - Voltage High

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Overview

Fault Code

SPN 3250 / FMI 3

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: DOC outlet temperature sensor is mounted on the exhaust pipe or DPF inlet canister, downstream of the Diesel Oxidation Catalyst, typically on the driver or passenger side of the vehicle frame rail near the aftertreatment assembly.

Common Symptoms

  • Check engine light illuminated. MIL lamp on. Scan tool shows DOC outlet temp reading maxed out or implausibly high (often 2000°F+ or pegged at max value). Possible reduced fuel economy. May see other aftertreatment-related faults logged due to bad temp data. DPF regeneration may be affected or seem incomplete. No noticeable drivability issues in most cases unless derate activates.

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

This fault typically does NOT inhibit active regeneration on its own since it is classified as a circuit fault rather than a rationality fault. However, the ACM uses DOC outlet temp to monitor regen efficiency and catalyst light-off. With bad data, the system may not accurately control regen fuel dosing or may log secondary faults. If additional aftertreatment faults set due to implausible temp readings, active regen may be inhibited. Recommend repairing promptly to ensure proper regen control and prevent soot accumulation.

System Interactions

SPN 3250 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 regarding aftertreatment harness chafing or sensor connector water intrusion issues common to your specific engine platform (Cummins, Detroit, Paccar, etc.).

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.

Deep-dive diagnostic page: SPN 3250 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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