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most likely you have a bad micro switch in the door latch assembly, you can order these but you need to know which one is bad, there are usually three, do you have a volt meter and know how to check continuity?
sure continuity is a test to check for a completed circuit,
http://www.applianceaid.com/images/ohmmeterhowto.JPG click this link to explain,
Most microwave door switches can be tested for continuity to see if the switch has failed. This can "fool" you sometimes. This switch looks fine, tested fine with an ohm meter and even made the "click" noise ok when we pressed on the actuator button.
Once we opened up the switch, it became clear this switch's contacts where being overheated and failing = new switch time.
This switch "looked" ok, "checked" ok with an ohm meter...but as you can see...it is not ok.
ok I will be careful - when I took the switch apart it seemed to work normally - when the switch depressed it broke the circuit