My Maytag microwave only heats food on the "add 30 seconds" setting.

My Maytag microwave only heats food on the "add 30 seconds" setting.

I have stainless steel Maytag microwave. Recently it only started heating food on the "add 30 seconds setting" The other heat settings do not work. Everything else works fine on the microwave (eg clock, turntable, vent, light). Can someone tell me what the problem is please?

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