SPN 205 FMI 6: Meaning and Fix
SPN 205 FMI 6 indicates the engine position sensor circuit has detected current above normal or a grounded condition. In practice, this code often appears after recent engine wiring harness repairs or exposure to moisture. Technicians frequently encounter this fault when the sensor connector becomes contaminated with oil or coolant, causing an intermittent short to ground. Left unresolved, the ECM may disable fuel injection, leading to a no-start condition.
Common Symptoms
- No Engine Start: ECM inhibits fuel injection due to invalid sensor signal, preventing engine cranking or firing.
- Erratic Tachometer: RPM gauge fluctuates or reads zero as ECM receives corrupted engine speed data.
- Reduced Power Mode: Engine enters derate with limited torque output to protect components from uncontrolled combustion.
- Check Engine Lamp: Amber warning lamp illuminates on dash; fault logged with active SPN 205 FMI 6.
Probable Causes
- Shorted Sensor Wiring: Insulation damage causing signal wire to contact ground or battery voltage, raising current draw.
- Failed Sensor Internally: Hall-effect or variable reluctance sensor shorted due to thermal breakdown or mechanical damage.
- ECM Driver Fault: Internal ECM pull-up resistor or current monitor circuit damaged from previous overcurrent event.
- Connector Contamination: Moisture, oil, or debris bridging pins in sensor connector, creating low-resistance path to ground.
Advanced Technical Analysis
The ECM monitors current flow through the engine position sensor pull-up circuit, typically 5V with a 10kΩ resistor. Under normal operation, current is limited to 0.5 mA. FMI 6 triggers when current exceeds 2.5 mA for 200 ms, indicating a short to ground or supply. The ECM’s analog-to-digital converter sees voltage near 0V or 5V, depending on fault type.
A short to ground forces the signal line below 0.8V, while a short to battery voltage (12V/24V) forces it above 5.5V. The ECM debounce timer prevents false triggers from noise. If the fault persists for three consecutive key cycles, the code becomes active. In MAN engines, this timer is extended to 500 ms to accommodate cold start variations.
Upon detecting this fault, the ECM immediately disables fuel injection and sets a torque derate to 30% of rated power. The engine may run on default camshaft-only timing if a second speed sensor is available. Deutz common-rail systems use this fallback to allow limp-home mode at 1200 RPM max, protecting the engine from runaway.
Long-term prevention includes dielectric grease on connectors and regular harness inspections. In one workshop case, a Mercedes-Benz OM471 repeatedly logged SPN 205 FMI 6 after engine washing. The root cause was water ingress at the 14-pin engine harness connector. Replacing the connector and cleaning with contact cleaner resolved the fault permanently.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor connector for corrosion, bent pins, or moisture. Inspect harness for chafing near engine block.
- Measure Resistance: Disconnect sensor; measure between signal and ground. Expect >10kΩ. Below 100Ω indicates short.
- Check Supply Voltage: With key on, measure sensor supply pin to ground. Should be 4.5-5.5V. Low voltage suggests ECM or harness issue.
- ECM Disconnect Test: Disconnect ECM connector; measure resistance from sensor signal pin to ground. If low, repair harness short.