SPN 723 FMI 6: Meaning and Fix
SPN 723 FMI 6 indicates the ECM has detected a current above normal or a grounded circuit on Engine Speed Sensor 2. This fault commonly appears after a forced DPF regeneration or a jump-start event that causes a voltage spike, shorting the sensor signal wire to chassis ground. The ECM immediately flags the circuit as overcurrent and disables the secondary speed input.
Common Symptoms
- Erratic idle: Engine hunts or stalls at idle due to missing secondary speed reference for fuel trim.
- Check engine light: Red or amber MIL illuminated with logged SPN 723 FMI 6 in active or inactive state.
- Reduced power: ECM derates torque by up to 40% to protect the engine from uncontrolled speed.
- No start condition: In dual-sensor configurations, engine may crank but fail to start if both sensors are compromised.
Probable Causes
- Shorted signal wire: Sensor signal wire chafed against engine block or chassis ground causing direct short.
- Failed sensor: Internal short in speed sensor 2 due to thermal breakdown or mechanical damage.
- ECM pin damage: Corrosion or bent pin at ECM connector for the sensor 2 input circuit.
- Harness water ingress: Moisture in the sensor harness connector creates low-resistance path to ground.
Advanced Technical Analysis
The ECM continuously monitors the current flow on the Engine Speed Sensor 2 circuit using a high-side sense resistor. When the current exceeds a calibrated threshold (typically 50-100 mA above normal), the microcontroller sets FMI 6 and disables the sensor input. This logic prevents damage to the ECM’s internal driver stage.
A grounded circuit causes the ECM to see a near-zero voltage on the signal line while current draw rises sharply. The ECM applies a debouncing timer of 0.5-1.5 seconds before latching the fault to avoid false triggers from transient noise. Real-world cases show this fault often occurs after engine washing.
Upon detecting a current fault, the ECM enters a fallback strategy: it relies solely on Engine Speed Sensor 1 (SPN 190) for speed data. If sensor 1 also fails, the ECM initiates a progressive torque derate down to 25% and limits engine speed to 1200 rpm. This protects the engine from runaway conditions.
Long-term prevention requires inspecting the sensor harness for chafing near the flywheel housing and using dielectric grease on connectors. Technicians frequently encounter this fault after replacing the ECM without transferring the original pin seals. Always perform a wiggle test with the ignition on to reproduce intermittent shorts.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check Engine Speed Sensor 2 connector and harness for chafing, corrosion, or water intrusion.
- Measure resistance: Disconnect sensor and ECM, measure signal wire to ground—should be >1 MOhm.
- Sensor substitution: Replace sensor 2 with a known-good unit and clear fault codes to verify repair.
- ECM pin check: Inspect ECM connector pin for sensor 2 circuit; repair any bent or corroded terminals.