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My apartment comes with a wall-mounted microwave (KitchenAid KHMC1857WSS) that I clean periodically. While doing so, I see this component mounted on the right-side inside the microwave. It clearly has a dark discoloration, looking like it was burnt or blown out. The component reminds me of sandpaper because it appears to have a gritty surface.

Sandpaper-like component inside microwave Close-up view of the component

I'm not sure what this piece is, but since it looks burnt and cracked am wondering if it needs to be replaced. Looking at the online user manual it's still unclear what this piece is. For instance, on page 6 of the above PDF, the section titled Replacement Parts, Accessories and Cleaning Supplies does not list any part name that pops out to me as being this piece.

What is this piece inside my microwave?

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  • That piece is just a cover plate. What's behind it? – LarryBud Jan 4 at 20:38
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    It has bigger problems than that burned panel. – blacksmith37 Jan 4 at 20:45
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    @blacksmith37 care to elaborate? – Trotski94 Jan 6 at 10:04
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    @Trotski94 I think he means that the door appears to be missing from the photos. – jammypeach Jan 6 at 13:08
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That's the waveguide cover. It's non-conductive, typically made of a mica/resin-based material.

Yours should be replaced. When you get that degree of carbonization (which should never happen, but eventually does, usually because food particles weren't cleaned off it promptly and left to attract electrical arcs on subsequent uses of the microwave), arcing becomes a lot more likely.

The arcing will eventually destroy the magnetron and possibly other components.

Fortunately, replacement waveguide covers are readily available. It can be a little tricky to get the old one out and put the new one in, depending on the microwave, but not at all beyond the usual household DIY-er. Yours appears to have a simple shape and be secured with screws, which should make replacement very simple.

You can either get a waveguide cover made explicitly for your make/model of microwave, or purchase a pack of sheets that can be used to cut one that fits your microwave. The former is more convenient, but of course costs somewhat more.

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    Except the microwave is the property of the apartment company, and is not OP's to play around with. Upside: it's also their responsibility, and they should ante up to fix it, unless it's plainly due to tenant neglect. – Harper - Reinstate Monica Jan 4 at 22:52
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    @Harper: "Except"? While it's possible the microwave belongs to the owner of the apartment, and it's possible that the rental contract precludes the tenant doing any maintenance on the microwave (you don't personally have any way to know any of that for sure), that changes nothing about my answer. It's still a waveguide cover, it's still damaged, replacement is simple and necessary, and involves acquiring either one specifically made, or cutting one to size from a sheet. Everything I wrote is true. What's the point of your comment? – Peter Duniho Jan 4 at 22:59
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    @PeterDuniho Possible? OP says so. And it's built-in; there's a picture right there. As far as "contract", no, that's decided in state law. Provided appliances must be maintained by the landlord, unless certain optional items are excluded by contract, but a built-in microwave would not be. Maintenance is divided into "trivial" (light bulbs) and "non-trivial" (fluorescent ballasts). You're arguing this falls in the "trivial" category. Maybe. I definitely don't agree on "making one". Other than that, your advice is sublime; so I won't DV it. – Harper - Reinstate Monica Jan 4 at 23:34
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    @Harper: "You're arguing this falls in the "trivial" category" -- I've done no such thing. I've simply outlined what it is, how it got that way, and what a person could do about it. I've made no statement regarding whether it's advisable/legal/etc. for the OP themselves to replace it, Your vehemence in this regard is surprising, to say the least. – Peter Duniho Jan 5 at 12:24
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    This is a great answer - it answers the question and shows how to proceed. The bit about whose responsibility this is for an appliance in a rental situation is really getting OT for DIY.SE. If OP wants to discuss that specific part, perhaps law.SE would be the right place? – Criggie Jan 6 at 7:50

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