There's a small button on your Auto transmission shifter that allows you to disable overdrive with your thumb, for example climbing a steep pass or pulling a trailer would be times you would want to do this to keep RPM's in the higher range for better power. My left knee sometimes accidentally pushes this button and shuts O/D off. The light would not be illuminated most of the time driving when you would want overdrive. The light may illuminate at startup just as many other lights do but it should be off unless you have specifically disabled O/D by pushing the button on the shifter. The light would then illuminate indicating the O/D is off. Hope that answers your question.