11 or 12 year old dishwasher. I started by siphoning out the water (my dishwasher is very close to the door to the back yard.). I found that the coarse screen seals had deteriorated away, and had allowed at least one napkin or paper towel into the pump screen. After I cleaned all that out (a stiff bristle brush and a mirror were helpful), the problem remained. So, more research indicated the piston with nut (valve) was to blame. Took it out and sure enough, it was worn enough to where it couldn't possibly work. So, I ordered both the valve and the screen from APP. They arrived very quickly. Not looking hard enough for the correct way to remove the spray arm (to replace the coarse screen), I managed to break a tab. So, the unit now drained correctly, but the lower spray arm would rise up and hit the lower rack, and not spin.
So, bought the Spray Arm Assembly. It too arrived very quickly. This time I found the proper procedure on YouTube, which was a pleasant surprise in that it was a piece of cake to accomplish. I thought I'd have to pull the unit out of the cabinet.
Five minutes (including a bit of cleaning before installing the new item), and I was done! Turned out, when I compared the new to the old, the old spray bar had 'warped' over the years, and it looked like it had regularly hit the lower rack.