SPN 171 FMI 4 — Ambient air temperature sensor low

SPN 171 identifies the parameter the control module is monitoring — ambient air temperature sensor low. FMI 4 is the failure mode the module logged against it. Both numbers matter: the same SPN with a different FMI is a different repair.

Regen logic and fan control affected.

• Shorted sensor wiring
• Failed sensor
• Damaged mirror-arm harness

• Compare reading to actual outside temp
• Inspect the mirror arm harness
• Ohm the sensor at temperature

Work the list in order and record the readings. Confirm the fault with live data before any part is ordered — a code names a circuit, never a failed component.

Generally safe to finish the trip

No driving restriction. Fix it at the next service.

• Replacing the sensor named in the SPN before checking its wiring, connector and ground.
• Clearing the code and handing the truck back without reproducing the operating condition that set it.
• Reading one code out of a set instead of finding which fault occurred first.

Get a shop involved when the fault comes with a red lamp or a shutdown timer, when the checks above all pass and the code returns, or when the repair needs an aftertreatment service tool, a forced regen, or engine internals. Nothing on this page is a substitute for OEM service literature or a qualified technician on the truck.

Enter this code in Blue Collar AI diagnostic support with the engine and the symptoms and you get the test sequence in order, per-truck history so repeat faults are visible, and a printable work order. Aftertreatment codes are covered in depth in the Aftertreatment Mastermind training. It is all in Truck Commander at $14.99 a month.