SPN 654 FMI 1: Meaning and Fix
This fault indicates the ECM detected the electrical current for cylinder 4 injector is below the normal operational threshold, often due to a short-to-ground or open circuit. Technicians commonly encounter this after replacing the ECM without reprogramming injector calibration codes, or after a wiring harness chafes against the engine block near the valve cover.
Common Symptoms
- Rough Idle: Cylinder 4 misfire causes uneven power strokes, noticeable at low RPM or during deceleration.
- Power Loss: Engine torque derate up to 40% under load as ECM disables the injector to protect the catalyst.
- Increased Smoke: White or gray exhaust smoke from unburned fuel passing through cylinder 4 without proper injection.
- Hard Start: Extended cranking time, especially in cold conditions, due to missing injection event in one cylinder.
Probable Causes
- Short-to-Ground: Injector wire insulation melted against exhaust manifold or engine block, causing low resistance path.
- Open Circuit: Broken wire inside harness jacket near cylinder 4 connector, often from vibration fatigue.
- Faulty Injector: Internal coil resistance below 0.5 ohms, measured at 0.2 ohms, triggering below-range detection.
- ECM Driver Failure: Internal MOSFET on injector drive channel 4 damaged by overcurrent or voltage spike.
Advanced Technical Analysis
The ECM monitors injector current via a precision shunt resistor and an analog-to-digital converter sampling at 1 kHz. When the commanded peak current (typically 25-30A for a piezo injector) falls below the calibrated threshold for three consecutive injection events, the fault sets. This logic prevents false triggers from transient noise.
Electrical analysis shows the below-range condition often results from a debouncing timer expiring. The ECM expects a minimum of 0.5 ohms load; a measured 0.1 ohm indicates a short. The driver circuit then enters a current-limiting mode to prevent thermal damage, pulsing the injector at reduced duty cycle.
Upon activation, the ECM initiates a cylinder-cut strategy: it disables fuel injection to cylinder 4 and increases rail pressure by 200 bar to maintain torque on remaining cylinders. This fallback protects the diesel particulate filter from raw fuel but reduces power by approximately 25% and increases fuel consumption by 15%.
Long-term prevention requires verifying injector return flow rates and coil resistance during every major service. In one real workshop case, a technician found the fault after a DPF regeneration caused thermal expansion that pinched the injector harness against the EGR cooler bracket. Re-routing the harness with spiral wrap resolved the issue permanently.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check harness from ECM to injector 4 for chafing, burns, or corrosion near cylinder head and valve cover.
- Resistance Measurement: Disconnect injector and measure coil resistance between pins; compare to spec (0.5-1.0 ohms typical).
- Continuity Test: Measure resistance from injector connector to ECM pin; should be below 2 ohms with no shorts to ground.
- Injector Swap Test: Swap injector 4 with a known good cylinder; if fault moves, replace injector; if not, inspect ECM driver.