Just a couple of ideas.
1. You have no pause in your main loop so you are polling the DS18B20 extremely fast. You may want to try a Pause 1000 (1second) in between you gosub start_Convert and your goto mainloop.
2. Your If/Then LED logic in start_Convert will only light the LED when the temperature is below 70 degF.
Code:if F_temp =>70 then low LED else high LED endif
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