SPN Fault Codes
Verified Factory Specs
SPN 111 FMI 3: Engine Coolant Level High Voltage / Open Circuit Protocol
Diagnostic trouble guide for SPN 111 FMI 3 (Coolant Level Sensor Circuit High). Surge tank probe testing, harness continuity, and derate prevention.
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SPN 111 FMI 3
High (Engine Derate Active)
System Overview: Cooling System
ECM detected an open circuit in the coolant surge tank level probe circuit.
Driver Symptoms
- Low Coolant light illuminated on dash despite full surge tank
- Torque derate applied to protect cylinder liners from cavitation/overheating
Probable Causes
- Corroded or broken 2-pin connector at bottom of plastic coolant reservoir
- Oxidized stainless steel coolant sensor probe tips inside the surge tank
- Low coolant volume below the cold fill line
Diagnostic Steps
- Verify actual coolant level in translucent surge tank is at the COLD MIN/MAX mark
- Disconnect 2-pin harness connector and place a fused jumper across pins: if code changes to FMI 4 (Circuit Low), replace coolant probe in tank
- If code remains FMI 3 with jumper installed, trace harness for broken wire back to ECM Pin 37
Sensor Wiring Pinout: 2-Pin Metripack: Pin A (Signal), Pin B (Sensor Ground Return)
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Technical Reference Citation: SAE J1939-71 Cooling System Diagnostic Standards