SPN 128 FMI 0CoolingMedium
Coolant Temperature Sensor Circuit Range/Performance
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 128 / FMI 0
Category
Cooling
Severity
Medium
Regen Inhibit
No
Inducement Risk
No
Component Location: Coolant temperature sensor is typically threaded into the engine block, cylinder head, or coolant crossover manifold near the thermostat housing.
Common Symptoms
- High coolant temperature warning lamp illuminated
- Engine derate or power reduction active
- Erratic or pegged coolant temp gauge reading
- Hard start or extended crank in cold conditions
- Fan clutch engaging at inappropriate times
- Intermittent or stuck coolant temp reading on dash display
Root Causes
- Coolant temperature sensor failed or out of calibration range
- Low coolant level causing air pocket at sensor tip
- Corroded or damaged sensor connector and terminals
- Wiring harness chafing or open circuit to sensor
- Faulty ECM/ECU not processing sensor signal correctly
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Derate Behavior
Mileage / Regen Notes
Active DPF regen may be inhibited or interrupted when SPN 128 FMI 0 is active, as ECM requires accurate coolant temp data to authorize and safely complete regen cycles.
System Interactions
SPN 128 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 128 FMI 0 — 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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