The contacts at the bottom get corroded over time. The coolant completes the circuit between the 2 prongs, so when it drops below sensor level, it can tell, so it trips the low coolant idiot light. Or if they're gunked up, it'll think the same thing. This is how the sensor in the Cobra works, so it's probably a similar setup. Pull it out and clean the contacts, whatever is on there should come off pretty easily.