SPN 174 FMI 6: Meaning and Fix
SPN 174 FMI 6 indicates the Engine Control Module (ECM) detected current above normal or a grounded circuit on the Fuel Temperature 1 sensor line. This fault commonly appears after a fuel filter replacement where wiring is pinched, or when a fuel heater element shorts internally. The ECM logs the fault and may trigger a derate to protect the injection system from unmonitored fuel temperatures.
Common Symptoms
- Check Engine Lamp: Amber warning lamp illuminates; fault may be intermittent if wiring chafes against engine block.
- Torque Derate: ECM reduces engine torque by up to 25% to prevent injection timing errors from unknown fuel temperature.
- Erratic Fuel Temp: Dash gauge or diagnostic tool shows fuel temperature reading at -40°C or pegged at full scale.
- Hard Cold Start: If sensor signal is lost, ECM uses default value, causing poor cold-start fuel metering in winter conditions.
Probable Causes
- Sensor Short Circuit: Internal short in the fuel temperature sensor, often caused by thermal stress or vibration fatigue.
- Chafed Harness: Wiring insulation worn through against engine bracket or fuel line, grounding the signal wire to chassis.
- Corroded Connector: Moisture ingress at the sensor connector creates a low-resistance path to ground, especially in high-humidity environments.
- ECM Internal Fault: Rare but possible: driver circuit failure inside ECM, often confirmed only after ruling out all external wiring.
Advanced Technical Analysis
The ECM applies a 5V pull-up resistor to the sensor signal line. Under normal operation, the resistive sensor creates a voltage divider. When FMI 6 occurs, the measured voltage drops below 0.2V, indicating a short to ground. The ECM debounces this condition for 0.5 seconds before logging the fault to avoid false triggers from electrical noise.
Using J1939 DM1 messages, the fault is broadcast with Suspect Parameter Number 174 and Failure Mode Identifier 6. The ECM continuously monitors the signal. If the short clears for more than 1 second, the fault becomes inactive but remains stored. Technicians should use a breakout box to measure signal voltage at the ECM pin while wiggling the harness.
Upon detecting a grounded fuel temperature signal, the ECM substitutes a default value of 40°C (104°F) for fuel timing calculations. This substitution can reduce fuel economy by 3-5% and increase particulate emissions. Some OEM calibrations also trigger a progressive torque derate starting at 10% after 5 minutes of continuous fault.
In Deutz TCD 2013 engines, this fault often follows replacement of the fuel pre-filter where the sensor connector clip is broken. Real-world data from MAN D2676 engines shows that 70% of SPN 174 FMI 6 cases are resolved by repairing chafed wiring near the engine lifting eye. Preventative dielectric grease application and harness routing inspection are recommended after any fuel system service.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor connector for corrosion, bent pins, and harness for chafing near engine brackets or fuel lines.
- Measure Resistance: Disconnect sensor; measure between signal pin and ground. A reading below 100 ohms indicates a short circuit.
- Check Supply Voltage: With key on, measure voltage between sensor signal and ground. Should be near 5V; if 0V, check ECM harness.
- Isolate the Circuit: Disconnect ECM connector; measure resistance from sensor signal pin to chassis ground. If low, repair harness short.