SPN 652 FMI 0: Meaning and Fix
SPN 652 FMI 0 indicates the ECM has detected the injector current for cylinder 2 is above the calibrated normal operational range, often due to a short-to-supply or high-resistance fault. This code commonly appears after a technician accidentally pinches the injector harness during valve cover reinstallation or after a forced DPF regeneration that thermally stresses the injector coil insulation.
Common Symptoms
- Rough Idle: Cylinder 2 misfire causes uneven engine rotation and vibration at idle speeds.
- Misfire Under Load: Insufficient or excessive fuel delivery from cylinder 2 leads to power loss during acceleration.
- Increased Exhaust Smoke: Unburned fuel from cylinder 2 produces white or black smoke from the tailpipe.
- Engine Derate Active: ECM reduces torque to protect the fuel system and aftertreatment components from damage.
Probable Causes
- Short-to-Battery: Injector 2 power wire chafed against engine ground or 12V supply rail inside harness conduit.
- Injector Coil Failure: Internal winding short due to thermal overload, vibration fatigue, or manufacturing defect.
- ECM Driver Damage: High current spike from injector fault has burned the internal low-side driver MOSFET in the ECM.
- Connector Corrosion: Moisture ingress at injector or ECM connector creates a low-resistance path to power.
Advanced Technical Analysis
The ECM monitors injector current via a precision shunt resistor and a high-speed ADC sampling at 20 kHz. When the measured current exceeds the calibrated threshold (typically 25 A peak for solenoid injectors) for more than 500 µs, the FMI 0 is set. This logic prevents thermal runaway of the coil and protects the driver stage.
Electrical analysis often reveals a debouncing timer (200 ms) that must be exceeded before the fault is latched. Intermittent shorts caused by wire chafing may only trigger the code when engine vibration increases at certain RPMs. A 100 MHz oscilloscope capture of the injector current waveform can confirm the overcurrent event shape.
Upon detection, the ECM immediately shuts off the injector driver for cylinder 2 and enters a torque derate of up to 40%. The fuel delivery is redistributed to the remaining cylinders, but cylinder imbalance may cause increased exhaust temperature and regeneration frequency. This is a safety fallback to prevent catastrophic injector or ECM failure.
Long-term prevention includes verifying injector harness routing per MAN TGL factory manual (tight bends < 15 mm radius cause insulation fatigue). In workshops, this fault is frequently seen after ECM replacement if the injector wiring was not replaced—old harness faults can destroy a new ECM within 10 engine hours. Always measure injector resistance cold (0.5–1.5 Ω) before reconnecting.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check injector 2 harness for chafing, pinch points, or melted insulation near the valve cover.
- Measure Resistance: Disconnect injector 2 connector; measure resistance between terminals. Compare to 0.5–1.5 Ω specification.
- Check for Shorts: With injector disconnected, measure resistance from each pin to engine ground. Must be >1 MΩ.
- Swap Injector Test: Swap injector 2 with another cylinder. If fault follows, replace injector; if not, inspect ECM driver.