SPN 4374 FMI 6: Meaning and Fix
SPN 4374 FMI 6 indicates the ECM detected current exceeding normal range on the DEF pump motor speed sensor circuit, or a short to ground. This fault often appears after a failed DEF thaw cycle where ice in the pump causes mechanical lock, drawing high current. Technicians also see this after replacing the ECM if the harness pin is misaligned, causing a direct ground path.
Common Symptoms
- Engine Derate: ECM reduces torque by up to 40% to protect aftertreatment system and prevent NOx exceedance.
- MIL Illumination: Malfunction Indicator Lamp and aftertreatment warning lamp activate immediately upon fault detection.
- No DEF Dosing: DEF pump stops delivering fluid, causing SCR system to disable and NOx conversion to cease.
- High Exhaust Temp: Exhaust temperature rises above 550°C due to unregulated NOx and passive regeneration attempts.
Probable Causes
- Shorted Motor Winding: Internal pump motor winding insulation breakdown causes direct short to ground or between phases.
- Chafed Harness: DEF pump harness rubs against frame rail or heat shield, exposing wire and creating ground contact.
- Corroded Connector: Moisture ingress at pump connector pins leads to electrolytic corrosion and low-resistance paths.
- ECM Driver Failure: Internal ECM output driver for pump motor supply shorts to ground, exceeding current limit.
Advanced Technical Analysis
The ECM monitors motor speed via a Hall-effect sensor integrated in the DEF pump. A current above normal (FMI 6) is triggered when the sensed current exceeds 8 A for more than 200 ms, indicating a short circuit. The ECM uses a debouncing timer to avoid false triggers from transient inrush during pump startup, but a sustained high current locks the fault active.
Electrically, the pump motor is a brushless DC type with three phase windings. A short in any winding to ground reduces resistance below 0.5 Ω, causing current to spike. The ECM detects this via a shunt resistor on the motor driver supply. Real-world cases show this occurs after DEF crystallizes inside the pump rotor, mechanically seizing it and causing the driver to overcurrent.
When FMI 6 sets, the ECM immediately cuts power to the pump motor driver to prevent thermal damage. It then logs the fault and initiates a torque derate of 25% after 1 hour of operation, escalating to 40% if the fault persists. The SCR system is disabled, and NOx emissions can exceed 5 g/kWh, risking compliance violations in regulated regions.
Long-term prevention requires inspecting the pump connector for corrosion and the harness for chafing, especially on trucks operating in salt-belt regions. Technicians often find that replacing only the pump without checking the ECM driver leads to repeat failures. Measuring motor winding resistance (should be 1.5–3.0 Ω) and checking for shorts to ground with a megohmmeter is critical before component replacement.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check DEF pump harness for chafing, melted insulation, or corrosion at the 6-pin connector.
- Resistance Check: Measure resistance between pump motor power pin and ground; should be >1 MΩ, not <10 Ω.
- Short Circuit Test: Disconnect pump, measure voltage at ECM driver output; should be 0 V with pump removed.
- Replace Pump: If harness and ECM pass tests, replace DEF pump assembly and perform system priming procedure.