This is usually caused by a shorted hood thermostat or a shorted triac on the control panel circuit board.
The hood thermostat is usually located on the floor of the microwave chassis behind the control panel. It should read "open" or "infinity" on a continuity or multimeter resistance test.
At our site, we now have a video available showing how to remove a typical control panel assembly in under 5 minutes.
If the hood thermostat is bad, You can order Samsung parts here
If you think the problem is on the control panel, we repair these nationwide by mail for $39.95 postpaid in most cases.
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William E. Miller