I replaced the filter cartridge that is located underneath the lower spray arm. The original filter cartridge had become completely blinded off, the filter media is a very fine mesh nylon fabric and it gets clogged up eventually with hard water scale and other non-soluble materials (including dishwasher detergent). When this filter gets plugged, too little water flows through to the pump to be recirculated during washing.
Replacement is relatively easy, except for the awkward position of your body (kneeling on the floor reaching into the bottom of the dishwasher. The lower spray arm is removed by just pulling up on it, it just snaps into place. The filter cartridge is held in place by 11 screws (!!) of three difference sizes, so keep track of what size screw goes in which location. The cartridge forms the upper half of the pump housing, so the impeller will be exposed when the cartridge is removed. There is a rubber gasket between the lower pump housing and the filter cartridge that will come off when you remove the cartridge. This should be re-seated on the lower pump housing before the new cartridge is installed. The only tool needed for replacement is a phillips screwdriver.