SPN 729 FMI 4ElectricalMedium
Intake Air Heater Relay — Voltage Below Normal
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 729 / FMI 4
Category
Electrical
Fault Type
actuator
FMI Category
electrical low
Severity
Medium
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Intake air heater — mounted in intake manifold or air inlet. Relay in fuse/relay box.
Common Symptoms
- Hard start in cold weather
- Intake air heater not activating
- Intake air heater relay fault active
Root Causes
- Intake air heater relay failure
- Wiring fault to relay coil
- Intake air heater element failure (high current draw)
- Fuse blown
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Relay failures common 300k–500k miles. Element failures can occur at any mileage.
System Interactions
SPN 729 FMI 4 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
TSB Reference: Cummins TSB 4021540 — Intake Air Heater System 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
ISX15: Heater element failure causes relay to fail due to overcurrent. Check element first.
Detroit
DD15: Check grid heater relay — high current relay, replace with OEM spec.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 729 FMI 4 — 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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