When do I know when my ecu has failed or my crank position sensor

 

When you get it professionally diagnosed. An ECM failure is rare. Crank sensor failure even more rare.

 

Are you suspecting the crank sensor because you scanned for OBD2 diagnostic codes? sometimes you can get codes set due to things other than a fault in the sensor ie a battery/voltage issue, faulty ground wire or wiring connection and so on. This is what the a professional diagnosis that follows a factory recommended triage will review.