Kashering a Microwave

A microwave heats food without a flame or a hot surface, so it is kashered differently from anything else in the kitchen: by steam.

The procedure

1. Clean thoroughly — all the walls, every hole, and the door — using cleaning agents.
2. Do not use it for twenty-four hours.
3. Boil water inside it, in an open vessel. Preferably put some cleaning agent in the water.
4. The water must actually boil, and the steam must spread across all the walls and condense onto the floor of the cavity as well.
5. Dry the walls with a clean cloth once they have cooled.
6. Replace the glass turntable with a different one.

Point four is the substance of the whole thing. The steam is the kashering agent, so it must genuinely reach every surface that absorbed. A brief burst that warms the water is not doing the job; the water has to boil long enough for the cavity to be full of steam.

And the turntable is replaced rather than kashered, which is worth noting since it is easy to overlook.

A microwave with a grill or browning element

This changes everything, and the reasoning is worth following.

Where a microwave has a browning function, a grill, or any similar feature, it is treated as an ordinary oven rather than as a microwave.

The reason: the browning element is a heating body that becomes hot and glows when operated. It absorbs from the vapour of the food beneath it in a way the cavity of a plain microwave does not. So it requires an oven's treatment — heating the element itself — and the steam method does not suffice.

Combination microwaves are now common, so check what yours actually has before choosing a procedure. If it browns or grills, you are kashering an oven. See kashering an oven.

Covers and containers

Much microwave use involves covered dishes, which changes what reaches the cavity. That is a relevant fact to bring to the question rather than to resolve yourself, since it affects what the walls absorbed in the first place.

What your microwave needs is a question for your rav.