no it doesnt light up with the key in the ACC, you have to be way low on fuel for it to even come on, haha, mine doesnt come on until my needle is below the E line, when it comes on it will come on brighter when braking or standing still, and go off when accelerating for about the first 20 miles, after that it pretty much stays on, thats when i know i only have a little bit to go. Another 10-15 miles and im out of gas, lol. But only way to see if it really works is to basicly run yourself out of gas, haha