Will a knock sensor not let car start if the is bad?

A bad knock sensor usually won't prevent a car from starting. While it's crucial for engine performance and efficiency, the car's computer (ECU) can usually default to a pre-programmed fuel map to allow the engine to crank and start. However, the car will likely run poorly, experience reduced power, and potentially throw a check engine light. The engine may also run rough, misfire, or even cause damage over time if the knock sensor issue isn't addressed.