I have a 2007 G6 GT Convertible Coupe (3.9l V6), and the coolant (no matter how many times I refill it) is always low. I can’t find a leak, and now the heat doesn’t work unless the vehicle is moving. I’m not sure if it’s the thermostat, if there’s a leak I’m missing, or if there’s just a small air pocket somewhere in the coolant lines. All I know, is that the heat works when the car’s moving, and it doesn’t when it’s stopped. It’s bad enough that I’ve spent more on coolant than I have on gasoline, so I’m guessing there’s a leak. I just need a helping hand. Anything would be much appreciated.