The knock sensor on a 1986 305 engine would not prevent the motor from firing. The function of the knock sensor is to detect the presence of engine knock, which is a condition that occurs when the piston makes contact with the cylinder head. When this happens, the sensor sends a signal to the engine control module (ECM), which then adjusts the ignition timing to reduce the risk of further knocking.
If the knock sensor is not functioning properly, the ECM will not be able to adjust the ignition timing properly, which can lead to engine damage. However, the knock sensor itself will not prevent the engine from firing.
