Reddy heater won't stay on. I have a 35,000 btu reddy heater that will fire up if I push the

Question

I have a 35,000 btu reddy heater that will fire up if I push the button, but it will only push heater for maybe 15 seconds or so and then shut down. If I push the button, it fires again, but merely repeats shutting down after 10, 15 seconds. What's wrong?