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SPN 158 FMI 18

Keyswitch Battery Potential — 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: The voltage the engine computer sees on its key-switch (ignition) input — how it knows the key is on and how healthy the switched power feed is. More on SPN 158.

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 dropping key-switch voltage can cause stalling, no-starts and a pile of unrelated codes. Fix power and grounds before chasing other faults.

Likely causes

  1. Weak batteries or poor charging
  2. Corroded or loose ignition feed, fuse or relay
  3. Worn ignition switch
  4. Aftermarket accessories spliced into the ignition circuit
  5. Poor ECM ground

Checks & fixes (easiest first)

  1. Load-test the batteries and check charging voltage
  2. Measure key-on voltage at the ECM ignition pin and compare with battery voltage
  3. Voltage-drop test the ignition feed and ECM ground under load
  4. Inspect fuses, relays and any aftermarket splices

FAQ

What does SPN 158 FMI 18 mean?

Keyswitch Battery Potential — 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 158 FMI 18 serious?

Low or dropping key-switch voltage can cause stalling, no-starts and a pile of unrelated codes. Fix power and grounds before chasing other faults.

What should I check first for SPN 158 FMI 18?

Load-test the batteries and check charging voltage; Measure key-on voltage at the ECM ignition pin and compare with battery voltage; Voltage-drop test the ignition feed and ECM ground under load.