The water distribution system in your dishwasher designed to operate only one spray arm at a time. During the first wash and first and second rinses, only the lower spray arm operates. You can learn more form the file attached to the reply.
It also can be not enough water in the dishwasher due to a clogged screen in the water inlet valve.
- The water inlet valve [PART]AP4321824[/PART]
Here are the breakdown diagrams and Replacement parts for FRIGIDAIRE FDB641RJS0 Frg(v2) / Dishwasher | AppliancePartsPros.com
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WATER DISTRIBUTION SYSTEM.pdf (93.3 KB)