I had the same problem - still getting F21 & SUD codes even after changing the pressure switch and the drain pump. No hose kinks or anything wrong with the drain lines. I finally took the drain lines all off - from the drain pump up to the plastic elbow near the top where it exited the washer, as well as the next line from that same elbow to where it drains into the house drain. We took them outside and ran water through them with a garden hose nozzle. There was some build up of gunk in those lines. I thought it was hardly enough to make much difference, but apparently it was, because after cleaning and reinstalling the lines, my wife has run 6 loads through the washer without the codes coming up again, so I'm hoping that it is finally fixed. Bottom line, apparently there was just enough restriction in the lines to slow the draining process enough to throw the codes. Hope this helps!