The symptom rarely names the part. A fireplace that lights and dies after thirty seconds is usually a thermocouple — but it can equally be a dust-fouled pilot, a weak valve, or an obstruction in the burner tube. Testing in sequence is what separates a $449 fix from replacing components the appliance never needed.
Long idle periods are the Dallas variable. Ten months of disuse in a warm climate does more harm than ten months of regular burning: sensors drift, pilot orifices clog, and insects nest in warm still spaces. Booking a service visit in autumn rather than waiting for the first cold front is the single cheapest thing an owner here can do. One thing to be clear about — unlike our masonry work, gas service and parts replacement carry no workmanship warranty from us; components carry the manufacturer's, and you hear that before the part is ordered. Venting matters too: a converted fireplace often has a flue sized for wood, and an oversized liner lets gas exhaust cool instead of drafting.