SPN 652 FMI 3: Meaning and Fix
This fault indicates the ECM has detected a voltage above normal or a short to high source on the cylinder 2 fuel injector drive circuit. In practice, this code commonly appears after a technician accidentally pinches the injector harness during valve cover reinstallation or after a rodent chews through the wiring insulation, causing an intermittent short to battery or a sensor supply line.
Common Symptoms
- Misfire at idle: Cylinder 2 fails to fire consistently, causing rough idle and visible exhaust smoke.
- Power loss: Engine torque derate activated by ECM to protect the injector driver circuitry.
- Check engine light: Red or amber MIL illuminated on dash, often accompanied by a stop engine lamp.
- Hard starting: Extended cranking time due to uneven fuel delivery from cylinder 2.
Probable Causes
- Shorted injector harness: Insulation breakdown causing direct contact between injector wire and battery voltage source.
- Faulty injector solenoid: Internal winding short to ground or to the injector body, raising circuit voltage.
- ECM driver failure: Internal short within the ECM output stage for cylinder 2 injector control.
- Corroded connector pins: Moisture ingress at the injector or ECM connector creating high-resistance or short path.
Advanced Technical Analysis
The ECM continuously monitors the injector drive circuit using a high-side current sense amplifier. When the voltage at the injector output pin exceeds a calibrated threshold (typically 5.5V for 12V systems or 11V for 24V systems) for more than 100 milliseconds, the ECM logs SPN 652 FMI 3. This debouncing timer prevents false triggering from transient spikes.
A short-to-high condition usually originates from the injector wire chafing against the valve cover or a chassis ground point that has become energized. Using an oscilloscope, technicians can observe the injector voltage waveform; a flat line near battery voltage confirms a hard short, while erratic spikes suggest intermittent contact with a sensor supply line.
Upon detecting this fault, the ECM immediately disables the cylinder 2 injector driver and activates a torque derate strategy. Fuel injection for cylinder 2 is cut entirely, and the ECM compensates by slightly increasing injection duration on the remaining cylinders. This fallback mode reduces engine power by 30-50% to prevent catalytic converter damage from unburned fuel.
Long-term prevention requires inspecting the injector harness routing and using dielectric grease on all connectors. In workshops, this fault often recurs after engine rebuilds if the injector wiring is not secured with original zip-ties. A permanent fix involves replacing the entire injector harness subassembly and verifying the ECM output with a breakout box before clearing the code.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check cylinder 2 injector harness for chafing, burns, or contact with sharp edges near valve cover.
- Resistance measurement: Measure injector solenoid resistance between pin 1 and pin 2; spec is 0.5-2.0 ohms at 20°C.
- Voltage check at ECM: Backprobe ECM connector pin for cylinder 2; key-on engine-off voltage must be below 0.5V.
- Harness isolation test: Disconnect injector and ECM; use megohmmeter to verify insulation resistance >1 MΩ to ground.