When a garbage disposal drains water properly on its own side of the sink, but water backs up into the opposite sink basin, the problem is typically caused by a blockage or grease buildup in the shared sanitary tee fitting or the main under-sink P-trap assembly. Because the two sink basins share a common drain connection downstream of the disposal, a restriction in the cross-pipe forces water to exit through the path of least resistance into the adjacent basin. Verification: Inspect and clean out the under-sink P-trap and branch drain pipes using a wrench and bucket.
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