I have three Dyna-Glo ventless NG hanging heaters. All in different buildings. Two work perfect but one lights as it should but after 2-5 minutes I hear a click and it goes out. I'm guessing the clicking I hear is the gas valve closing. All of them have oxygen sensors on them but this happens even if I have the garage doors open or closed. All of them have what I believe to be a flame sensor by the pilot light and without taking them apart to measure the sensor appear to be the same distance from the pilot light. It's frustrating to light the heater and come back a hour later thinking the garage will be warm only to find out the heater went out. Anybody ever worked on these or have any idea what I should look for. Thanks Gerald