Is the receiver and radio working? With out the engine running is everything working? Do the wheels turn and the throttle servo work. You said it only goes straight so I am guessing the steering is not working. So I would say some sort of radio problem, bad receiver, bad crystal or crystal not in all the way, bad TX, not hooked up properly. I all so would recommend checking the batteries again. Check them with a volt meter as I have gotten bad brand new batteries.