If the engine doesn't crank over immediately as you run the car for a while and then stop it, then this could be caused by a weak battery that is not being properly charged by a bad alternator. After sitting the car for 15 minutes, battery accumulates enough charge to crank the engine again and once you run the car around with a bad alternator, all the current required for spark generation is coming from the battery so it gets exhausted by the time you turn the car off. If the Crankshaft Position Sensor is bad then the engine will still crank but won't start. This sensor doesn't restrict the starter motor from cranking the engine.
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