What are the symptoms of SPN 520 FMI 31?

Amber or red engine warning light illuminated. Engine protection mode active. Elevated coolant temperature gauge reading. Reduced engine power or torque. Engine overheat warning message on dash. Radiator fan running continuously at high speed. Coolant temperature exceeds normal operating range

What causes SPN 520 FMI 31?

Engine coolant temperature exceeding programmed threshold due to cooling system restriction or low coolant level. Faulty or stuck-closed thermostat causing inadequate coolant circulation. Clogged or restricted radiator reducing heat dissipation capacity. Failed or degraded water pump with reduced flow rate. Air pockets or trapped air in cooling system after service. Radiator fan clutch failure or cooling fan not engaging properly. Collapsed lower radiator hose restricting coolant flow. ECT sensor out of calibration or drifted, reporting false high temperature. External coolant leak reducing system pressure and coolant volume. Blown head gasket causing combustion gases entering coolant

How do I diagnose SPN 520 FMI 31?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 520 FMI 31 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 520 FMI 31Engine Protection

Engine Coolant Temperature - Condition Exists

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Overview

Fault Code

SPN 520 / FMI 31

Common Symptoms

Amber or red engine warning light illuminated
Engine protection mode active
Elevated coolant temperature gauge reading
Reduced engine power or torque
Engine overheat warning message on dash
Radiator fan running continuously at high speed
Coolant temperature exceeds normal operating range

Root Causes

Engine coolant temperature exceeding programmed threshold due to cooling system restriction or low coolant level
Faulty or stuck-closed thermostat causing inadequate coolant circulation
Clogged or restricted radiator reducing heat dissipation capacity
Failed or degraded water pump with reduced flow rate
Air pockets or trapped air in cooling system after service
Radiator fan clutch failure or cooling fan not engaging properly
Collapsed lower radiator hose restricting coolant flow
ECT sensor out of calibration or drifted, reporting false high temperature
External coolant leak reducing system pressure and coolant volume
Blown head gasket causing combustion gases entering coolant

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.

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Derate Behavior

Power Derate

Yes

Mileage / Regen Notes

Active DPF regeneration will be inhibited or suspended when coolant temperature exceeds safe operating limits. High ECT may trigger engine derate and prevent regen initiation to avoid additional thermal load on the engine.

System Interactions

SPN 520 FMI 31 interacts with the following systems and may trigger cascading faults if not resolved promptly.

ECM communication — J1939 network faults can affect all connected modules

Engine protection — ECM may trigger derate to prevent damage

Aftertreatment — engine protection derates can affect regen strategy

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.

Related Faults in This Category

SPN 1569-31Engine Protection Derate SPN 100-1Oil Pressure Low SPN 190-0Engine Speed High SPN 111-4Coolant Level Sensor Low

OEM System Resources

Cummins Fault Codes Detroit Diesel Fault Codes PACCAR Fault Codes Volvo Fault Codes Freightliner Fault Codes Mack Fault Codes

ASE Training Resources

Why ASE Certification Matters How ASE Tests Work ASE Study Tips ASE vs AI Diesel Tech Certification Paths Get Started with ASE

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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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