What are the symptoms of SPN 105 FMI 4?

Engine derate possible — ECM may limit fueling without valid intake air temp. Intake air temperature gauge reads minimum or zero. Cold start fueling may be incorrect — ECM cannot calculate proper enrichment. Turbocharger over-boost protection may be affected. Active regen timing may be affected without valid intake temp data

What causes SPN 105 FMI 4?

Intake manifold temperature sensor shorted to ground — signal wire grounded. Intake manifold temperature sensor internal failure — sensor output stuck low. Intake manifold temperature sensor connector damage — signal pin shorted to ground pin. Wiring harness short to ground — signal wire contacting chassis or engine ground. ECM analog input circuit fault — internal ECM failure (rare). Intake manifold temperature sensor failed open — thermistor open circuit causing low voltage

How do I diagnose SPN 105 FMI 4?

Full step-by-step DVOM tests, electrical procedures, and connector pinout data for SPN 105 FMI 4 are available inside the AI Diesel Tech guided diagnostic session. The AI walks you through every test — one step at a time.

SPN 105 FMI 4Air/BoostModerate

Engine Intake Manifold Temperature — Voltage Below Normal or Shorted Low

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Overview

Fault Code

SPN 105 / FMI 4

Common Symptoms

Engine derate possible — ECM may limit fueling without valid intake air temp
Intake air temperature gauge reads minimum or zero
Cold start fueling may be incorrect — ECM cannot calculate proper enrichment
Turbocharger over-boost protection may be affected
Active regen timing may be affected without valid intake temp data

Root Causes

Intake manifold temperature sensor shorted to ground — signal wire grounded
Intake manifold temperature sensor internal failure — sensor output stuck low
Intake manifold temperature sensor connector damage — signal pin shorted to ground pin
Wiring harness short to ground — signal wire contacting chassis or engine ground
ECM analog input circuit fault — internal ECM failure (rare)
Intake manifold temperature sensor failed open — thermistor open circuit causing low voltage

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Derate Behavior

Power Derate

Possible

Mileage / Regen Notes

IAT sensor open circuit failure increases above 300,000 miles. Harness short to ground risk increases with age and heat cycling.

System Interactions

SPN 105 FMI 4 interacts with the following systems and may trigger cascading faults if not resolved promptly.

TSB Reference: Cummins TSB 4021801 — Intake Manifold Temperature Sensor Diagnosis; Detroit TSB 14-019 — IAT Sensor Circuit Diagnosis

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.

OEM Notes

Cummins

ISX/X15: IAT sensor open circuit (FMI 4) is common on high-mileage ISX engines. Replace sensor and inspect connector. INSITE IAT sensor test required.

Detroit

DD15: IAT sensor harness short to ground near charge air cooler is a known issue. Inspect harness at CAC area. DDDL intake temp test required.

PACCAR

MX-13: IAT sensor signal wire short to ground is a known failure mode. Inspect connector and harness carefully. DAVIE4 IAT test required.

Volvo

MP8: IAT sensor open circuit failure is common on MP8. Replace sensor and inspect connector seal. PTT intake temp test required.

Related Faults in This Category

SPN 5246-31SCR NOx Efficiency Low SPN 4364-18DEF Dosing Below Normal SPN 3251-0DPF Differential Pressure High SPN 157-18Fuel Rail Pressure 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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