SPN 5357 FMI 3: Meaning and Fix
SPN 5357 FMI 3 indicates that the Engine Control Module (ECM) has detected a voltage above normal or a short to high source on the fuel injection quantity monitoring circuit for multiple cylinders. This fault commonly appears after a forced DPF regeneration when the ECM checks injector feedback voltages and finds an abnormal rise, often due to chafed wiring near the exhaust manifold. Technicians frequently encounter this fault after replacing the ECM without properly verifying the injector harness integrity, leading to recurring diagnostic trouble codes.
Common Symptoms
- Rough Idle: Engine idles erratically with visible shaking and inconsistent RPM due to faulty injection quantity signals.
- Power Loss: Significant reduction in engine power under load as ECM limits fuel delivery to protect components.
- Excessive Smoke: Black or white smoke from exhaust caused by incorrect fuel metering across multiple cylinders.
- Hard Starting: Extended cranking time or failure to start because ECM cannot verify correct injection quantity.
Probable Causes
- Short to Battery: Injector harness wire insulation melted or chafed, contacting a 12V or 24V source directly.
- Faulty Injector: Internal short circuit within one or more injectors causing elevated voltage on the common feedback line.
- ECM Damage: Internal ECM driver circuit failed high, often due to voltage spikes from incorrect jump-starting.
- Connector Corrosion: Moisture ingress at the ECM or injector connectors creating a conductive path to voltage.
Advanced Technical Analysis
The ECM continuously monitors the injector feedback voltage on a dedicated sense line for each cylinder bank. When the voltage exceeds a calibrated threshold (typically 5.0V ±0.3V for a 5V reference system), the ECM sets SPN 5357 FMI 3. This monitoring occurs every 10 ms during engine operation. A persistent high condition for more than 2 seconds triggers the fault. The ECM uses a debouncing timer to filter transient spikes, but a sustained high voltage indicates a hard electrical fault.
Electrically, a short to battery voltage (12V or 24V) on the injection quantity feedback circuit will pull the sense line above the ECM’s internal pull-up voltage. The ECM’s analog-to-digital converter reads values above 110% of the expected full-scale range. This can occur if an injector solenoid wire rubs through against the engine block or a harness bracket. Common locations include where the harness crosses the valve cover or near the exhaust manifold.
Upon detecting this fault, the ECM enters a safety fallback mode: it disables fuel injection to all affected cylinders and commands a severe torque derate (often to 30% or less). The ECM also logs a freeze frame with engine speed, load, and battery voltage at the moment of fault. This data is critical for diagnosis. The ECM will not allow a restart if the high voltage condition persists, to prevent injector or ECM damage.
Long-term prevention requires inspecting the injector harness for chafing at common wear points, especially after engine repairs. Technicians should measure the feedback voltage at the ECM connector with a multimeter (key-on, engine-off) to confirm a short. Real-world example: a fleet truck repeatedly set this code after DPF regen; the root cause was a melted injector harness near the EGR cooler. Replacing the harness and using heat-resistant sleeving resolved the issue permanently.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect injector harness for chafing, melt marks, or pinched wires near exhaust and EGR components.
- Voltage Measurement: With key on, measure voltage at ECM connector pin for injection quantity feedback; should be below 5V.
- Isolate Injectors: Disconnect injector connectors one by one while monitoring voltage to isolate the shorted circuit.
- Check ECM Grounds: Verify ECM ground circuits are clean and tight; poor grounds can cause false high voltage readings.