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SPN 171 FMI 4

Ambient Air Temperature — Voltage below normal or shorted low.
The sensor circuit voltage is lower than it should ever be, usually near zero.

What the SPN covers: Outside air temperature, used for cold-start strategies and engine protection. More on SPN 171.

What FMI 4 usually points to: Signal wire shorted to ground, a lost 5 V reference, or an open signal wire on a pulled-down circuit. More on FMI 4.

Severity

LOW — usually OK to keep running

Minor; may affect cold-start and fan behavior.

Likely causes

  1. Wiring or connector fault in this circuit (most common for electrical FMIs)
  2. Faulty sensor (often in the mirror or bumper)
  3. Wiring fault
  4. Network data missing from the module that owns the sensor

Checks & fixes (easiest first)

  1. Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins
  2. Check the reference voltage, ground and signal at the component per OEM pin-out
  3. Compare reading to actual outside temperature
  4. Inspect sensor and connector
  5. Check whether the value comes from another module over J1939

FAQ

What does SPN 171 FMI 4 mean?

Ambient Air Temperature — voltage below normal or shorted low. The sensor circuit voltage is lower than it should ever be, usually near zero.

Is SPN 171 FMI 4 serious?

Minor; may affect cold-start and fan behavior.

What should I check first for SPN 171 FMI 4?

Inspect the connector and harness for this circuit: corrosion, water, chafing, backed-out pins; Check the reference voltage, ground and signal at the component per OEM pin-out; Compare reading to actual outside temperature.