Detailed guide for SPN 171 FMI 4
Back to top ↑What SPN 171 FMI 4 means
SPN 171 identifies ambient air temperature.
FMI 4 indicates voltage below normal or a signal shorted toward a low source.
What you may notice
The outside-temperature reading may appear extremely low or may not match actual ambient conditions.
Depending on the engine and vehicle, incorrect ambient temperature data can affect cold-weather, idle-shutdown or aftertreatment strategies.
What to check first
Inspect the sensor connector and wiring for corrosion, water intrusion, chafing or a short to ground.
Check the sensor supply, ground and signal circuits using the manufacturer's wiring diagram.
On some trucks the sensor is mounted in a location exposed to road salt, moisture and debris, making connector and harness inspection especially important.
Important
A low electrical signal does not automatically mean the outside temperature is actually low. Diagnose the circuit before replacing components.
Mechanic community notes for SPN 171 FMI 4
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