SPN 111 FMI 3 — Coolant Level Sensor Circuit Voltage Above Normal
Level-sensor high-voltage codes still start with physical coolant verification. False low-coolant warnings with full tanks after connector corrosion are the most documented pattern. The FSM enforces physical-first gate to prevent unnecessary sensor replacement and missed real leaks.
Fault Overview
SPN 111 FMI 3 is a coolant level sensor circuit high voltage or open class fault. Open signal, failed level sensor, unplugged header connector, and corroded terminals are the primary causes. False low-coolant warnings with full tanks after connector corrosion are documented patterns. Replacing the level sensor without verifying physical coolant level first is the most documented misdiagnosis pattern — a true low coolant condition produces the same code as a sensor circuit fault.
Engine & Platform Coverage
Symptoms
- •Coolant level sensor open
- •False low coolant warning
- •Level sensor high voltage
- •Coolant lamp on with full tank
- •Low coolant warning with no visible leak
Possible Causes
- 1.Open signal — open signal circuit producing high voltage reading
- 2.Failed level sensor — failed level sensor producing open circuit
- 3.Unplugged header connector — unplugged surge-tank header connector producing open circuit
- 4.Corroded terminals — corroded terminals producing open circuit
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Step 1 — Physical Level Verification: Verify physical coolant level first. A true low coolant condition produces the same code as a sensor circuit fault. Do not condemn the sensor before visual level proof. Inspect the surge tank and radiator for actual coolant level. A true low coolant condition requires leak diagnosis, not sensor replacement.
- 2Step 2 — Connector Inspection: Inspect the surge-tank sensor connector. False low-coolant warnings with full tanks after connector corrosion are a documented pattern. Inspect the connector for corrosion, coolant contamination of pins, and chafe at the tank bracket. Clean or repair the connector if corrosion or contamination is found.
- 3Step 3 — Circuit Open and High-Voltage Checks: Check for open-circuit conditions and high-voltage conditions in the sensor circuit. Verify reference voltage, ground, and signal circuit integrity at the sensor connector. An open signal circuit produces a high-voltage reading at the ECM input.
- 4Step 4 — Sensor Test and Replacement: Test the sensor per OEM procedure if the circuit is confirmed good. Replace the sensor if the circuit is good and the sensor fails the OEM test. Do not replace the sensor before circuit proof.
- 5Step 5 — Verify After Repair: Verify coolant level sensor operation after repair. Confirm SPN 111 FMI 3 does not return. Confirm the low-coolant warning clears after repair.
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
2 steps locked- 1Repair connector or harness if open circuit is confirmed.
- 2Clean or replace connector if corrosion or contamination is confirmed.
- 3Replace level sensor if circuit is good and sensor fails OEM test.
- 4Address true low coolant condition if physical level is confirmed low.
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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Summary
Level-sensor high-voltage codes still start with physical coolant verification. The AI Diesel Tech FSM enforces physical-first gate to prevent unnecessary sensor replacement and missed real leaks.