Will a crank sensor keep the car from starting?

Yes, a faulty crank sensor can absolutely prevent a car from starting. The crank sensor tells the engine control module (ECM) the crankshaft's rotational speed and position. Without this information, the ECM can't accurately time the fuel injection and ignition, making it impossible to start the engine. The symptoms can range from a complete no-start condition to a difficult start, especially when the engine is cold.