SPN 1327 FMI 6: Meaning and Fix
SPN 1327 FMI 6 indicates the Engine Cylinder 5 Combustion Metrics sensor circuit has detected current above normal or a short to ground. This fault commonly appears after a forced DPF regeneration when excessive vibration or heat damages the cylinder-specific ion sense wiring harness near the exhaust manifold. Technicians frequently encounter this after replacing the ECM without verifying the injector harness integrity.
Common Symptoms
- Rough Idle: Engine runs unevenly at idle due to cylinder 5 misfire rate exceeding 5% threshold.
- Reduced Power: ECM initiates torque derate to protect aftertreatment system from unburned fuel.
- Excessive Smoke: White or blue smoke from exhaust caused by incomplete combustion in cylinder 5.
- Check Engine Lamp: Amber warning lamp illuminates immediately when misfire rate surpasses 2% over 100 cycles.
Probable Causes
- Sensor Short: Ion sense harness chafed against engine block causing a direct short to ground.
- Injector Fault: Stuck open or closed injector on cylinder 5 alters combustion ionization current.
- ECM Damage: Internal driver failure in ECM due to voltage spike from alternator overcharging.
- Wiring Corrosion: Moisture ingress at cylinder 5 connector pin causing high resistance and false current reading.
Advanced Technical Analysis
The ECM monitors the ion sense signal from cylinder 5 every engine cycle via a high-speed ADC. When the current exceeds 2.5 mA for more than 10 consecutive cycles, the firmware sets FMI 6. This threshold is derived from SAE J1939-71 and Bosch ED17+ application notes for misfire detection. The debouncing timer prevents false triggers during transient load changes.
A short-to-ground scenario typically shows a steady 4.5 mA current reading on the diagnostic tool, regardless of engine RPM. This differs from a normal ion signal which varies between 0.8 mA and 2.2 mA. The ECM compares the cylinder 5 value against the average of cylinders 1-4 to confirm the fault. A deviation greater than 30% triggers the code.
Upon activation, the ECM engages a two-stage derate: first 25% torque reduction after 50 cycles, then 50% after 200 cycles if the fault persists. Fuel injection to cylinder 5 is cut after 300 cycles to prevent catalyst damage. This strategy aligns with Mercedes-Benz OM471 factory protocols for protecting the SCR system from unburned hydrocarbons.
Long-term diagnosis requires checking the cylinder 5 ion sense circuit resistance between the sensor return and ECM pin. A resistance below 100 ohms confirms a short. Real-world workshop data from Deutz TCD 12.0 engines shows 70% of these faults are resolved by replacing the cylinder-specific wiring harness section, not the sensor or ECM. Always perform a resistance test before component replacement.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check cylinder 5 harness for abrasion near exhaust manifold and injector connector for corrosion.
- Resistance Test: Measure resistance between sensor signal pin and ground; should be >1 MΩ, not <100 Ω.
- Injector Balance: Run cylinder cutout test to isolate if injector mechanical fault causes the high current reading.
- ECM Pin Check: Verify ECM connector pin for cylinder 5 ion sense is not pushed back or damaged.