Sensor or mechanical? Isolation tool

Two very different families of fault can affect the crankshaft area, and they're often confused. On one side, a mechanical failure (shells, mains, play) shows up as noise, vibration and falling oil pressure — the engine runs but suffers. On the other, a crank speed sensor (CKP) fault is purely electronic: it cuts ignition and injection, throws a fault code and prevents starting, with no abnormal noise. Answer the questions below to steer the diagnosis toward the right family before opening anything. This tree points the way; it doesn't replace a scan-tool code read or a measurement.