SPN 633 FMI 5 indicates the fuel injection control valve circuit is open or drawing insufficient current — an open/low-current class fault on the inlet metering valve (IMV), fuel control actuator (FCA), or equivalent rail control valve. Open connector, broken wire, failed valve coil, and poor ECM driver connection are the primary causes. Hard start or no-start after the valve connector is not fully locked during pump service is the most documented pattern. Replacing the pump assembly before proving the control valve circuit is the most common misdiagnosis.

Inspect the control valve connector at the pump or rail assembly — heat and vibration damage are common at this location. Verify connector is fully locked. Check for broken wires in the harness run from ECM to valve. Inspect ECM connector for backed-out or corroded terminals on the driver circuit.

Measure coil resistance against OEM specification — an open coil will read infinite resistance. Verify continuity from ECM driver pin to valve connector. Confirm no short to ground or power before retesting. Avoid repeated actuation commands into an open circuit. After circuit is proven clean, verify command response — the valve should respond to commanded changes in rail pressure.