My 15 year old build in fireplace is giving me problems for the first time. Pilot stays lit but main flame goes out after a few minutes. I have cleaned everything including surface of thermocouple which I suspect. Wiring seems good as I can jumper the ignition bypassing the switch and even that wont work when it shuts down. I'm open to suggestions but if it is the thermocouple is that something I can simply replace? I see where it unscrews from the main assembly. I don't know what is inside a thermocouple or how it triggers the mechanism so I am leery of just taking it off. Please advise. Thank you.