Microwave oven Diode Burnout ?

Two easy things to check:
1. Is the unit getting enough cooling air? Check to see if vents are partially or fully blocked and make sure that the required clearances are met.
2. (Less likely) but, check the incoming line voltage. If you live in North America, it should be 120V plus or minus 5%.
When I bought my house, the builder offered a built-in unit. I declined for reasons similar to those you mentioned. Too much customization and hassle when the unit goes bad. Even though I gave up counter space for a free-standing unit, when the last microwave I owned went bad, I replaced it for a new unit, on sale for $60. Microwave ovens are almost a commodity item these days.

No help here, I replace 'em.
Our house is 20YO with the microwave built into the copper range hood. When it went, about 5 years back, I found that it wasn't tough to find a replacement, though it helps if you like black, white, or stainless. ;-) While the built-ins with exhaust fans aren't cheap (I paid about $600 with a convection-oven that's never been used), they are available and seem to be a standard size. One may have to do some "invention" to install the new one though.

The most common microwave failures are the Fuse and or the fuse holder. If you have a diode that keep failing then it must be either to much current or voltage going to it.
Check all connectors and replace any that are over heated and discolored. I have found most of the time that the fuse holder even though it looks fine starts failing and you get a high voltage drop across it and that raise the current in everything else to give the same watts with less voltage.

Find a suitable replacement that incorporates a heatsink in the package. That should extend its life beyond that of the original or OEM replacements.
Their "OEM" replacement *may* be a device that they call for on many models, but may not have been the same device that was originally installed in your unit.

Right. Like you have "worked on" numerous MW ovens.

have a diode You're an idiot, and you put shame on your stolen nym, asshole. Joe Bloe knows far more than you do. have found most you get a high voltage watts with less voltage.
Total mental retardation.

Why not get one from a higher rated unit and give that a try ? (The previous suggesting regarding ventilation is very valid too!)

1) If you aren't particular about colour, you can replace them for about $120/US. You can get a "pretty good" model for less than $200. Remember that the uWave also replaces the over the stove fan unit including the light. They have extra thermostats inside to shut things off before anything gets too hot. They will also automatically turn on the fan if the over gets things too hot. I don't know about the "builder's models" but the one comsumers buy are of a good quality.
2) My firs uWave lasted over 25 years. It was a good quality unit for the time with extra features (for the time).
3) Just so you know, an over the oven uWave is partly supported by the cabinet that's on top of it and partly by a stamped steel plate which is attached to something strong in the wall. The power comes from an outlet wired into the cabinet and the cord runs through a hole you make in the cabinet. The unit I installed in my rental is about 6 years old. The one in my home is about 1 year old. My old one that lasted 25 years had a voltage doubler and transformer for the magnitron supply. I would think one would replace the diode and the doubling cap at the same time.
Frankly, I agree with others than when it gives trouble you replace it. That's what I would do even with the somewhat fancy model ($270) my wife got me to put in.
There are FOUR basic "colors" and one should suit you. They are: White, Black, Stainless Steel, and some variation of "Yellow." The latest version of "yellow" is an "off white". Black or Stainless goes with anything. If you have very old appliances, the current "yellow" may not work for you.

Exactly my point, Dimbulb.

you have a diode have found most and you get a high voltage watts with less voltage. Your usual "contribution" to the Usenet community, Dimbulb?

I'm sorry that I can't agree about replacing the whole unit. For that amount of money, surely it must be more economical and rewarding to repair the old one and avoid unnecessary waste/ landfill, as long as it's not rusty inside and the hinges aren't worn. I do agree about replacing the diode & capacitor as a pair next time, though. Still not expensive. If it still fails, then perhaps even the magnetron as well, the time after. Or even pay good money and take it to a workshop which will offer a guarantee on the work! Still cheaper than a complete equivalent replacement oven.

Actually, the replacement magnetron is very likely quite expensive, and would be the point at which choosing a new unit is a better solution.
Would it not be better to get the schematic for it, and try to find out why the diode is blowing?
The fix is likely a beefier diode. Inexpensive, and the right solution.