I have a real piece of junk microwave.. a Westinghouse 3501.. This was actually the second one i got, after returning the first one.. i wish i would've taken the time to return this one, also.. Anyway, after warming food at full power for 3 minutes, the power will completely shut off.. i open the door, i let it cool out in there, and about a minute later it turns back on again.. then i proceed to start warming my food again, and it usually shuts right off after about another minute.. this is driving me crazy.. it ends up taking the better half of an hour just to microwave 3 baked potatoes!!
If the time from START to shut off were EXACTLY 3 minutes, then I would suspect a sensor problem or corrupted code in the microprocesor. But since that time sounds more random, it sounds heat-related, and I would suspect a failing magnetron or a failure in the cooling system. If the cooling fan is not running while the oven is cooking, this failure can occur, and can destroy the magnetron in short order. A cracked magnet (has the oven been dropped?) or other problem can cause a magnetron to go bad and overheat. If it's under warranty, I would definitely contact the manufacturer (see your manual) to find an authorized local servicer. If it's out of warranty, I would balance the cost of a new microwave versus a bench diagnosis fee of $25 or more or an in-shop repair cost of maybe $80 or more.
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