Need help! Our Kenmore Ultrawash dishwasher is about 5 year old. Recently it does not clean the dishes on the upper rack at all. It cleans the dishes on the lower rack somewhat but not as good as it used to be. I did a little bit homework. I disassemble the spray arm and the food chopper part. I cleaned any debris that I can see out (it was not a lot) and checked the "check value" (it looks ok). No improvement. Then we did an experiment, We trick the machine to operate while keeping the door open. We noticed that the upper spray arm produce nearly no water jet stream, while the lower spray arm also produce relatively low water jet stream (doesn't reach the upper rack at all). So we suspect it is the impeller problem. Is it possible? Any insight?

Thanks!!