SPN 1633 FMI 4HVAC
Cab Interior Temperature - 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 1633 / FMI 4
Category
HVAC
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Cab interior temperature sensor is typically mounted in the dash panel or overhead console area to measure cabin air temperature.
Common Symptoms
- HVAC control issues or erratic temperature regulation
- Cab temperature sensor fault code active
- Climate control system defaults to fixed temperature setting
- Blower fan operates at maximum speed continuously
Root Causes
- Failed cab interior temperature sensor Damaged wiring harness or connector to temperature sensor Corroded or loose connector pins at sensor Short to ground in sensor circuit Faulty HVAC control module
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Derate Behavior
Power Derate
No
System Interactions
SPN 1633 FMI 4 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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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 1633 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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