SPN 171 FMI 3Air IntakeMedium
Ambient Air Temperature Sensor - Voltage Above Normal or Shorted High
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Overview
Fault Code
SPN 171 / FMI 3
Category
Air Intake
Fault Type
sensor
FMI Category
voltage high
Severity
Medium
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Ambient air temperature sensor mounted in the air intake path - typically on the air filter housing, air intake duct, or front of the vehicle in the airstream. Location varies by OEM and vehicle configuration.
Common Symptoms
- Ambient air temperature reading stuck at maximum or above normal range
- HVAC system may behave incorrectly - AC or heat not responding to actual ambient temp
- Engine cold-start strategy may be affected - ECM uses ambient temp for cold-start fueling
- Possible companion faults: SPN 171/4 (ambient temp voltage low)
- Temperature gauge or display showing incorrect ambient temperature
Root Causes
- Ambient air temperature sensor signal wire shorted to voltage
- Ambient air temperature sensor internal failure - signal circuit shorted high
- Sensor connector moisture intrusion - water bridging signal and reference pins
- Damaged wiring harness - chafed signal wire contacting power source
- Sensor mounted near heat source - reading artificially high due to radiant heat
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
Random failure. Connector corrosion increases with road exposure and moisture.
System Interactions
SPN 171 FMI 3 interacts with the following systems and may trigger cascading faults if not resolved promptly.
TSB Reference: Cummins TSB 4021756 - Ambient Air Temperature Sensor 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
X15: Ambient air temperature sensor is used for cold-start fueling and HVAC control. Incorrect reading can affect cold-start performance in extreme temperatures.
Detroit
DD15: Sensor mounting location near the air filter housing - inspect for heat soak from nearby components.
PACCAR
MX-13: Sensor connector is exposed to road spray - moisture intrusion is common. Inspect connector boot seal.
Volvo
MP8: Ambient air temperature sensor is integrated with the HVAC system - incorrect reading affects cab temperature control.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 171 FMI 3 — 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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