The crankshaft sensor is in your vehicle's engine and controls the ignition switch timing and makes sure the crankshaft is in the correct position and rotating at the proper speeds to provide the engine with adequate power. The crankshaft sensor took the place of the distributor in newer-model vehicles equipped with distributor-less ignition systems. It performs essentially the same job as the distributor in older model vehicles.
Out of Time
If the crankshaft sensor is not reading properly or working correctly, the vehicle will misfire and generally act as if it is not running properly because it will be out of time. If the crankshaft, camshaft and ignition are not working precisely in-sync, the vehicle's timing will not be correct and it will idle roughly, run poorly and may even periodically stall out.
Failure to Start
If the crankshaft sensor stops working, and the crankshaft is not in the correct position for the ignition to fire off, then the vehicle may not start up at all, or may start only to immediately die again. If a vehicle is not receiving the correct spark from the ignition, it will not run.
Check Engine Light
Your "check engine" light will come on if your crankshaft sensor is not working properly. These sensors are predominantly in newer vehicles, so you can hook the car's computer up to a code reader and scan for error codes. The crankshaft sensor has a number of codes that it will present if there is a problem with it. This is the easiest way to diagnose a problem with your car's crankshaft sensor.