Microwave ovens are built to start only when the door catch has been engaged. If the door is open, the machine won't start the cooking process. If your microwave has a broken door catch, the unit will not cook. You will need to replace the broken catch with the appropriate Whirlpool microwave part, or whichever brand you own. The door catch microwave part catch runs under a dollar for some models. To install this microwave part, you will need to disassemble the door and replace the door catch. Pay careful attention to how you remove the faulty door catch and you should have no trouble installing the new one.