SPN 4193 FMI 6: Meaning and Fix
This fault indicates the ECM detected excessive current on the Engine Coolant Pump Outlet Temperature sensor circuit, typically caused by a short to ground or internal sensor failure. Technicians often see this code after a coolant leak has soaked the wiring harness, or following an engine rebuild where the sensor connector was damaged. The ECM continuously monitors the signal voltage; a reading below 0.2 V for more than 0.5 seconds triggers FMI 6. This condition prevents accurate coolant temperature reporting, affecting engine protection strategies.
Common Symptoms
- Dash Warning Lamp: Red or amber stop engine lamp illuminates immediately upon fault detection.
- Erratic Temperature Gauge: Coolant temperature gauge on dash may read maximum or minimum value.
- Engine Power Derate: ECM reduces torque by up to 25% to protect engine from potential overheating.
- Failed DPF Regen: Active regeneration may be inhibited due to missing coolant temperature input.
Probable Causes
- Sensor Shorted Internally: Failed NTC thermistor inside sensor creates a direct low-resistance path to ground.
- Harness Chafing: Wires rubbed through against engine bracket or coolant pipe causing short to ground.
- Connector Corrosion: Moisture ingress at sensor connector leads to electrolytic bridging of pins.
- ECM Internal Fault: Rare but possible shorted pull-up resistor or damaged input channel on ECM.
Advanced Technical Analysis
The ECM supplies a 5 V pull-up voltage through a fixed resistor to the sensor signal line. When the NTC thermistor resistance decreases with rising temperature, the voltage drops proportionally. FMI 6 triggers when the ECM measures the signal voltage below 0.2 V for a debounce period of 500 ms, indicating a short to ground. This logic prevents false triggering from transient noise.
A current above normal condition means the circuit is drawing more than the expected 2-5 mA typical for a normal sensor. A short to ground can create a current path exceeding 20 mA. The ECM’s internal current-limiting protection may activate, but the fault is latched. Technicians must confirm the short is not intermittent by performing a wiggle test on the harness while monitoring the signal.
Upon detecting FMI 6, the ECM substitutes a default coolant temperature value of 95 °C for engine protection. This forces the cooling fan to run continuously and may enable a torque derate of 25% after 30 seconds. In some MAN and Deutz engine calibrations, the ECM also disables exhaust gas recirculation to prevent thermal overload. The derate remains until the fault is cleared.
Long-term prevention includes inspecting the coolant pump outlet sensor harness routing, especially near the fan shroud and exhaust manifold. A common workshop scenario is finding the harness melted against the turbocharger heat shield after a previous repair. Using factory-specified Deutsch DT connectors with dielectric grease is recommended. After repair, perform a test drive with J1939 data logging to verify stable readings between 20-100 °C.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor connector and harness for corrosion, chafing, or melted insulation near heat sources.
- Measure Sensor Resistance: Disconnect sensor; measure resistance between signal and ground. Should be > 1kΩ at 20 °C.
- Check for Short to Ground: With sensor disconnected, measure resistance from signal pin to engine ground. Must be > 10 MΩ.
- Verify ECM Input: Reconnect sensor; back-probe ECM pin; voltage should be 0.5-4.5 V depending on coolant temperature.