SPN 651 FMI 4 indicates the cylinder 1 injector circuit voltage is below the valid minimum — a short circuit or low-signal class fault. Internal injector short, harness short, oil-bridged terminals, and pinched loom after service are the primary causes. Avoid repeated actuation into a short — ECM driver stress is a real risk.

Inspect insulation rub-through on sharp edges and valve-cover corner damage — these are the highest-probability short locations for injector circuits. Check for oil-filled connectors. Avoid repeated actuation into a short.

Disconnect the cylinder 1 injector and check resistance — below OEM minimum supports internal short. Check short-to-power and short-to-ground with injector disconnected. If short clears with injector disconnected, the injector is internally shorted. If short persists, inspect harness at valve-cover edge and bracket chafe points. Repair circuit before installing a new injector if harness is at fault.