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SPN 111 FMI 3Heavy truck
Moderate

SPN 111 FMI 3SPN 111 FMI 3 — Voltage above normal / shorted high

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Check soon

— FMI 3 is electrical (voltage high/short)—not proof the tank is empty. Still address it soon: false level data can trigger protection the same as a real low.

What does this severity mean?

General guidance only — not a substitute for a professional inspection or official roadside / out-of-service criteria (e.g. CVSA OOSC) in your jurisdiction.

Part of system: Engine cooling & overheat protection

With SPN 111 FMI 3, prove the circuit before you condemn the part named in the scanner text. SPN 111 FMI 3 means voltage above normal or shorted high on the coolant-level sensor circuit—an electrical fault, not a confirmed empty tank. The signal wire may be shorted to battery/ignition voltage, the sensor internally failed open in a pull-up circuit, or the connector is contaminated. Diagnose with a wiring diagram: measure reference, signal, and ground at the sensor with the connector connected and disconnected before pouring coolant or replacing the tank. Do not confuse FMI 3 with FMI 1 low-data faults. Discriminate among signal wire shorted to battery or ignition voltage, open sensor element on a circuit that floats high, and damaged connector shorting pins inside the housing using voltage drop and connector integrity. Verify by measure signal voltage key-on engine-off at the sensor connector, then disconnect the sensor and see if the voltage collapses as specified; replace coolant level sensor after electrical confirmation only when the wiring path checks out clean.

Parts that may be involved

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Vehicles often affected

Trucks with corroded surge-tank connectors after salt exposure or prior harness repairs that shorted signal to power.

Typical symptoms

  • SPN 111 FMI 3 sets at key-on with a full, cold surge tank
  • Signal voltage pinned near battery voltage on a meter
  • Fault unchanged after topping off coolant
  • Connector pins show green corrosion or pushed-out terminals
  • Other sensors on the same 5 V reference may also read high

Likely causes

  • Signal wire shorted to battery or ignition voltage
  • Open sensor element on a circuit that floats high
  • Damaged connector shorting pins inside the housing
  • Incorrect jumper or prior harness repair splicing power into signal
  • ECM input fault after wiring and sensor test good

Recommended checks

  1. 1

    Measure signal voltage key-on engine-off at the sensor connector

  2. 2

    Disconnect the sensor and see if the voltage collapses as specified

  3. 3

    Megger or continuity-check signal to power and ground circuits

  4. 4

    Repair the shorted harness section or replace the pigtail

  5. 5

    Replace the level sensor only after the wiring proves sound

DIY vs professional

DIY-safe: inspect connector pins for corrosion. Pro: voltage/continuity diagnosis to the ECM pins is the reliable path before replacing the sensor.

See also

Related terms

Content reviewed by a heavy-duty technician. About our editorial approach