Is a 20 year old dishwasher that was never opened still any good?

I put one of these in my rental about five years back, a dishwasher that had sat in a basement still crated for probaly fifteen years. It ran fine mechanically, but the door gasket had gone stiff and it wept water around the door on the second wash. New gasket, problem gone. So the machine itself was good, the rubber was not.

Here's what I'd do in your shoes. First, find the model number, it's ussually on a sticker on the edge of the door once you open it. Punch that into a parts site and see if gaskets, hoses and pump seals are still stocked. For common brands from that era they usually are, because makers keep parts around for a long time and aftermarket copies exist for the popular models. If you can't find anything, that tells you something about future repairs too. Before you install it in the rental, bench test it. Hook it to a water supply and drain somewhere you can watch, lay a towel under the door, and run a full cycle or two. That's how you find out if the seals held up without flooding anyone's kitchen. Also check inside the box for mouse evidence, since that chewed corner is a clue. Wiring insulation is something rodents love, so have a look at the junction box area and the cord before you power it up.

Plan on replacing the inlet hose and drain hose regardless, they're cheap and old ones under pressure are the thing that fails at 2am. The door gasket too. After that, if the pump runs quiet and it drains and cleans, you're likely to get your few decent years out of it. I wouldn't sink serious money into a motor or control board if one of those goes, at that point a scratch and dent new unit is a better spend.

One more thing, detergents and rinse aids from that era can degrade but that's a non issue, just use fresh stuff. The real question is just whether the rubber survived, and the bench test answers that in an afternoon.

This is kind of funny timing because my dad pulled an old oven out of his storage shed last summer that had never been turned on, and it worked fine except the knobs had gotten sticky and crumbly, same idea I guess, the plastic and rubber stuff ages even when it's just sitting there. One thing I keep wondering about though. When they build these things at the factory don't they run water thru them to test before boxing? So there coud be leftover water sitting in the pump and hoses for 20 years, wouldn't that get gross or stink up the first few loads?

Agree with the bench test advice, that's exactly how it played out for me. I hooked up an old crated washer my brother-in-law was getting rid of, ran it in the driveway off a garden hose adapter before it went inside, and caught a stuck float switch that would've overflowed the tub. Twenty bucks and ten minutes of fiddling and it was fine. Sounds dumb but the driveway test saved me from finding that out the hard way in the kitchen. Also second the part about not chasing big repairs on one this old, mine ran another four years in the apartment before I swapped it out.