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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?

ChiefOfGxBxL
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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.

Peter Duniho
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    I agree it looks like the waveguide cover that prevents debris from causing reflections, + – Ed Beal Jan 04 '20 at 21:48
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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 04 '20 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 04 '20 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 04 '20 at 23:34
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    I'll double check the lease agreement, but regardless I will be putting in a maintenance request for this now that I know the proper name for this piece. I remember the waveguide cover being cracked and damaged when I moved in, although it may have gotten worse during my occupancy. – ChiefOfGxBxL Jan 05 '20 at 04:25
  • Kitchenaid seems to want 50 bucks for a replacement. https://www.kitchenaidparts.com/Shop-For-Parts/a6b121c11/Kitchenaid-Microwave-Cap-Lid-Cover-Parts the DIY route should be much cheaper. https://www.banggood.com/145-x-98mm-Microwave-Oven-Universal-Mica-Wave-Guide-Cover-Sheet-p-1038939.html – Jasen Jan 05 '20 at 06:00
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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 05 '20 at 12:24
  • The irony here is that "non conductive" is exactly the property the cover loses in that condition ... and that is the root of the problem. – rackandboneman Jan 05 '20 at 16:16
  • @Jason 50 bucks! outrageous price like for and accessory for an x-Phone.. – quetzalcoatl Jan 05 '20 at 18:33
  • @PeterDuniho I understand his vehemence - I have to reread your answer to ensure that there is no wording like "you should replace it" etc. After initial read, I also got an impression that you are advising the OP to do it hiimself. There is no such statement in your answer, but something there in the text made me feel like it was there. Assuming that most people read once and go, I somewhat agree with Harper that it's better to include a warning. I know we're in "DYI.stackexchange" but then, I know of an apartment owner that tried charging tenant after the tenant replaced a fuse in a DVR.. – quetzalcoatl Jan 05 '20 at 18:38
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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 06 '20 at 07:50
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    @Harper-ReinstateMonica while I sympathize with your comment and agree with you that the OP probably needs to have the apartment management fix it, I think this answer hits the nail on the head and questions about financial responsibility doesn't matter here (or does it?). Someone finding this answer on the internet probably has this problem with their own microwave so this answer applies to a broad audience. – Cave Johnson Jan 06 '20 at 20:19
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    @KodosJohnson I just think when the first four words of a question specifically call out a landlord-owned device, a sentence is called for to say "don't fix other people's stuff without their permission". I don't think that's a huge or outrageous critique, and I think Peter's reaction is over-the-top. Noting several here are passersby here but regulars on SO, that may be a cultural difference. – Harper - Reinstate Monica Jan 06 '20 at 22:15
  • @Criggie I don't agree. I think it's not only reasonable, but dutiful to remind posters of common but non-obvious newbie pitfalls, like "turn the power off", "get a big panel" or "avoid GFCI on the fridge circuit". Again, may be a cultural thing, but in our defense we deal in an area where it's easy to make mistakes that kill. – Harper - Reinstate Monica Jan 06 '20 at 22:21