 I don't know which one would be number one, low in oil or low in coolant. Either one, if they're low enough, they can definitely cause damage to your vehicle. So if you're planning on going on a trip, check to see if your oil changes do. And even if it's not due, make sure the level's okay. And then make sure that the coolant level's okay. And then on some cars, you can check the transmission. Some you can't.