The sensor is just a grounding switch. for a light. If the wire is disconnected from the sensor the light should be off (it should come on with the key on and the engine not running, as a test.) You could stick something in the end of the wire and ground it out to the block or frame and see if the light comes on. (Remember, no Teflon tape on the sensor threads. You can use some other thread/gasket sealant.) You could have a bad sensor, or one for a gauge.