It is not recommended to use a microwave's oven function without a functioning mica plate or waveguide cover. The mica plate or waveguide cover is a protective component that covers the opening where microwaves enter the cooking chamber. Its purpose is to prevent food particles, grease, and moisture from entering the sensitive electrical components of the microwave.
Without a mica plate or waveguide cover, there is a risk of food particles or moisture getting into the inner workings of the microwave, which can lead to electrical arcing, damage to the microwave, and even potential safety hazards.
If your microwave's mica plate or waveguide cover is damaged or missing, it is advisable to have it replaced by a professional technician or contact the manufacturer for assistance. It is important to ensure that your microwave is in good working condition and that all safety components are intact before using it.