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Fault Code Reference

SPN 3510 FMI 18AftertreatmentMedium

DEF Pressure Sensor - Low

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Overview

Fault Code

SPN 3510 / FMI 18

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: DEF pressure sensor is integrated into the DEF supply module, mounted on or near the DEF tank, typically on frame rail behind cab.

Common Symptoms

  • Low DEF pressure reading on scan tool (below 75 psi), DEF dosing system not injecting or under-dosing, MIL/CEL illuminated, possible DEF warning lamp, NOx conversion efficiency codes may also be present, white crystalline deposits around DEF injector if system has been running lean on dosing, intermittent or no DEF spray visible during injector activation test

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

This fault typically does NOT inhibit active DPF regeneration directly. However, if DEF dosing is disabled due to low pressure, SCR inlet temperatures may be affected and the system may limit regen frequency to protect the catalyst. Extended low DEF pressure can lead to high NOx and eventual SCR efficiency codes which CAN inhibit regen. Verify dosing is functional before performing regen.

System Interactions

SPN 3510 FMI 18 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 Cummins QuickServe for ISB/ISX/ISL DEF pressure sensor updates. Check PACCAR for MX-13 DEF module bulletins. Check Detroit Diesel for DD13/DD15 aftertreatment TSBs. Cold weather DEF system bulletins may apply if freeze-related.

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 3510 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.

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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