Cleaned screens on old water valve, cleaned lower filter-screen, and checked for obstructions. F20 code persisted, so I ordered water valve, flow meter and pressure switch. Replaced water valve first, and ran 1 full cycle without F20 code. The F20 code returned on the 2nd test cycle, so I changed the flow meter. All good so far after 2-3 test cycles and a full load of laundry.
The mounting tab on the water valve is shorter than the original part, so I had to expand the mounting slot on the chassis to secure the water valve. Four stars rather than five for that reason.