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Fault Code Reference

SPN 1761 FMI 19Medium

DEF Tank Level - Data Erratic

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Overview

Fault Code

SPN 1761 / FMI 19

Severity

Medium

Regen Inhibit

No

Inducement Risk

No

Component Location: DEF tank level sensor is integrated into the DEF tank sending unit assembly, mounted inside the DEF tank typically located on frame rail behind cab or under cab on driver side.

Common Symptoms

  • DEF tank level gauge jumping erratically or displaying inconsistent readings. Dash gauge may show full then empty or random percentages. DEF level warning lamp may illuminate intermittently. Possible false low DEF warnings despite full tank. Driver complaints of gauge bouncing while driving. Stored inactive codes for DEF level high/low alongside this active fault.

Root Causes

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Derate Behavior

Mileage / Regen Notes

This fault alone typically does not inhibit active DPF regeneration. However, if the erratic data causes the system to interpret DEF level as critically low or empty, the ACM may limit DEF dosing which can indirectly affect SCR efficiency and potentially trigger regen-related derates. Monitor for companion SCR efficiency codes. Regen will proceed normally as long as DEF dosing system remains functional.

System Interactions

SPN 1761 FMI 19 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: No confirmed universal TSBs — verify in OEM service portal for specific chassis. Cummins has issued service letters regarding DEF tank sender contamination and cleaning procedures. Detroit Diesel/Freightliner has bulletins on DEF header/sender unit connector corrosion. Check Paccar for DEF tank assembly updates on Kenworth/Peterbilt platforms. International/Navistar has released guidance on DEF system diagnostics including sender unit replacement criteria.

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 1761 FMI 19 — 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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