AI Diesel Tech

Fault Code Reference

SPN 3031 FMI 18Medium

DEF Tank Temperature Sensor

Run Full DiagnosisView Related Faults

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 3031 / FMI 18

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: DEF tank temperature sensor is integrated into the sending unit assembly mounted on top of the DEF tank, typically behind cab on frame rail.

Common Symptoms

  • DEF gauge reading erratic or stuck at one value. DEF tank heater not activating in cold weather causing DEF to freeze. MIL lamp illuminated. Possible DEF quality fault codes setting due to incorrect temperature compensation. SCR efficiency codes may follow. Driver may report white crystalline buildup around DEF tank cap in cold weather due to heater malfunction.

Root Causes

  • [object Object] [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.

Start Guided DiagnosisSee How It Works

Derate Behavior

Mileage / Regen Notes

FMI 18 (data valid but below normal range) typically does not directly inhibit active regeneration. However, if the ECM cannot accurately determine DEF temperature, it may limit DEF dosing which can cause SCR efficiency to drop. This can lead to secondary codes that will inhibit regen. If DEF is actually frozen due to heater not activating (because ECM thinks tank is warm), dosing will be suspended until thaw is detected.

System Interactions

SPN 3031 FMI 18 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: Temperature sensor TSBs

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.

Deep-dive diagnostic page: SPN 3031 FMI 18 — 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.

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 3031 FMI 18

The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.

Run Full DiagnosisView Related Faults