Home › Codes › SPN 158 › FMI 1

SPN 158 FMI 1

Keyswitch Battery Potential — Value too low (most severe).
The reading is valid but far below the normal operating range, at the most serious 'too low' level.

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 1 usually points to: A real under-condition such as very low oil pressure or low fuel pressure, or a sensor that has drifted low. More on FMI 1.

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 1 mean?

Keyswitch Battery Potential — value too low (most severe). The reading is valid but far below the normal operating range, at the most serious 'too low' level.

Is SPN 158 FMI 1 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 1?

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.