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The low oil sentry does just that, it tells you when your oil is low and cuts off the ignition so that you do not burn up your engine.
These very often go bad (it is just a sensor) Normally to test them I simply unhook the one wire that is going to the sensor (located towards the bottom of the engine block) If everything starts working properly again I know that the sensor has gone bad. Sometimes you can clean them with carburetor cleaner and bring them back to life, other times you will need to replace the sensor.
Because the sensor is malfunctioning. You can unplug the sensor and remove and clean it or replace it