Detroit Diesel SPN 171 FMI 3 – Ambient Air Temperature Sensor Voltage High (Meaning & Fix)

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Detroit Diesel SPN 171 FMI 3 indicates that the ambient air temperature sensor is reporting a voltage signal above the expected range. This can affect fuel management and emissions performance.

Symptoms

  • Check engine light illuminated
  • Incorrect temperature readings
  • Reduced fuel efficiency
  • Possible aftertreatment warnings

Cause

This fault is commonly caused by wiring issues, short circuits, damaged connectors, or a failed ambient temperature sensor.

Can You Keep Driving?

Driving may be possible temporarily, but continued operation with low fuel pressure may cause engine stalling or additional damage.

Recommended Action

Inspect wiring and connectors for damage. Test the ambient air temperature sensor and replace if necessary.

What does Detroit Diesel SPN 171 FMI 3 mean?

SPN 171 FMI 3 indicates that the ambient air temperature sensor is reporting a voltage higher than the expected range.

What causes SPN 171 FMI 3?

Common causes include a faulty ambient temperature sensor, shorted wiring, damaged connectors, or electrical issues in the sensor circuit.

Can I keep driving with SPN 171 FMI 3?

Driving is typically possible, but inaccurate temperature readings can affect fuel management and emissions performance.

Will SPN 171 FMI 3 cause a derate?

In most cases it does not immediately cause a derate, but unresolved sensor faults can contribute to performance or emissions-related issues.

How do I fix SPN 171 FMI 3?

Inspect wiring and connectors for damage, test the ambient air temperature sensor, and replace it if voltage readings remain outside specification.

Related codes

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Important Notice

The information provided above is for general informational purposes only and should not replace official OEM service procedures. Always consult the manufacturer’s service manual or a qualified diesel technician before performing repairs. Operating a vehicle with active fault codes may result in further damage.