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SPN 171 FMI 2

Ambient Air Temperature — Data erratic, intermittent, or incorrect.
The signal jumps around, drops in and out, or disagrees with other information the controller has.

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

What FMI 2 usually points to: Loose or corroded connectors, chafed wiring, a failing sensor, or two redundant sensors that do not agree. More on FMI 2.

Severity

LOW — usually OK to keep running

Minor; may affect cold-start and fan behavior.

Likely causes

  1. Faulty sensor (often in the mirror or bumper)
  2. Wiring fault
  3. Network data missing from the module that owns the sensor

Checks & fixes (easiest first)

  1. Compare reading to actual outside temperature
  2. Inspect sensor and connector
  3. Check whether the value comes from another module over J1939

FAQ

What does SPN 171 FMI 2 mean?

Ambient Air Temperature — data erratic, intermittent, or incorrect. The signal jumps around, drops in and out, or disagrees with other information the controller has.

Is SPN 171 FMI 2 serious?

Minor; may affect cold-start and fan behavior.

What should I check first for SPN 171 FMI 2?

Compare reading to actual outside temperature; Inspect sensor and connector; Check whether the value comes from another module over J1939.