SPN 171 FMI 16Engine Protection
Ambient Air Temperature - Data Valid But Above Normal (Moderately Severe)
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 171 / FMI 16
Category
Engine Protection
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Ambient air temperature sensor is typically mounted in the front of the vehicle behind the bumper or grille area away from engine heat sources.
Common Symptoms
- Check Engine Light or MIL illuminated
- Engine derate or power reduction
- High coolant temperature warnings
- Excessive radiator fan operation
- Reduced fuel economy
- Difficulty achieving DPF regeneration
- Ambient temperature reading above 160°F (71°C) displayed
Root Causes
- Ambient air temperature sensor reading high due to heat soak from engine compartment Faulty ambient air temperature sensor providing incorrect signal Sensor placement too close to heat source such as exhaust or turbocharger Wiring harness issue causing intermittent high signal to ECM ECM calibration issue or software requiring update
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Derate Behavior
Power Derate
No
System Interactions
SPN 171 FMI 16 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
OEM System Resources
ASE Training Resources
Diagnostic Engine v7.0
Related Diagnostic Articles
Deep-dive diagnostic page: SPN 171 FMI 16 — 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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