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SPN 168 FMI 18

Battery Potential / Power Input 1 — Value too low (moderately severe).
The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

What the SPN covers: Voltage the engine computer sees on its main power supply. More on SPN 168.

What FMI 18 usually points to: A developing under-condition that often precedes a derate. More on FMI 18.

Severity

MEDIUM — fix soon

Low or unstable voltage can cause many unrelated codes and no-starts. Fix power first.

Likely causes

  1. Weak batteries
  2. Loose or corroded battery cables
  3. Charging system fault (alternator)
  4. Bad ECM power or ground connection

Checks & fixes (easiest first)

  1. Load-test batteries
  2. Clean and tighten battery and ground cables
  3. Check charging voltage at the alternator and at the ECM
  4. Voltage-drop test the ECM power and ground circuits

FAQ

What does SPN 168 FMI 18 mean?

Battery Potential / Power Input 1 — value too low (moderately severe). The reading is below normal at a middle warning level — more serious than FMI 17, less than FMI 1.

Is SPN 168 FMI 18 serious?

Low or unstable voltage can cause many unrelated codes and no-starts. Fix power first.

What should I check first for SPN 168 FMI 18?

Load-test batteries; Clean and tighten battery and ground cables; Check charging voltage at the alternator and at the ECM.