SPN 174 FMI 3 indicates the fuel temperature sensor circuit voltage is above the normal maximum — an open circuit or high-signal class fault. Open signal wire, failed thermistor, unplugged connector, and corroded terminals are the primary causes. A connector left loose after filter housing service is the most documented pattern. Fuel system teardown for what is actually an open temp circuit is the most common misdiagnosis.

Inspect sensor pigtail near filter housings and fuel rails — these locations are subject to fuel contamination and heat damage. Check connector for fuel contamination that may have corroded terminals. Verify connector is fully locked after any filter or housing service.

An open circuit typically causes the ECM to read an extreme cold fuel temperature regardless of actual conditions. Compare the PID reading against ambient temperature at cold soak — if the reading is implausibly cold at ambient temperature, the circuit is open. Measure thermistor resistance against OEM specification at a known temperature.