Model: WFW8410SW01 6 years old
It seems that every other wash load yields the F20 error. I ran through the diagnostic cycle without any errors. Then ran the cleaning cycle: It filled with water and drained, then sat silent and counted down the remaining minutes from 41 to 1, then paused on the 1 until I had to cancel.
Things I already checked:

• unplugged, replugged electricity
  
• removed drain pump, cleaned it out
  
• vacuumed out the drain hose
  
• removed the water inlet hoses
  
• cleaned filter screen
  
• verified water pressure is good and water is clean from the hoses
  
• pressure hose is clear
  

When the F20 happens, it could be near the beginning of the wash cycle, or near the end. I'm usually left with "clean" clothes, but damp and have to be wrung out. Hitting the cancel button seems to drain the unit just fine.

The manual says it's the pressure switch. The hose connected to the switch looks fine. It's "protected" with a foam sleeve. What's my next step?