2003. The unit fired up, but after a lengthy time, perhaps 20 seconds or more, and there was that loud whoop sound of a lot of gas igniting at once. It was heating, I thought so I left it and upon my return about an hour later, it wasn't burning. It displayed the diagnostic code IGN. The top was not hot so after it fired up. it must have shut down quickly. What does the IGN code indicate and how can I address it? Any help is greatly appreciated.