So this is really baffling... Our GE Profile dishwasher is having an issue with dirty water backing into it while it's off. We've had the dishwasher for years with no issues, it just recently started doing this. And the water is really gross. High drain loop is correct (and always has been with no issues). We figured a check valve somewhere in the pump must have gone bad, and the water is somehow siphoning or backing up from the kitchen sink? (We have no disposal FYI). So we added a secondary check valve in the drain line, thinking that would surely solve the problem (as opposed to trying to replace or fix an interior one in the pump?). This worked for a day or so, but alas the problem has returned. Totally baffled how drain water can be backing into the dishwasher with the new check valve in place. And high loop in drain line. Driving us crazy. Any ideas???
Fill kitchen sink with water and drain it while observing high drain loop if it is translucent. Or disconnect drain hose from Dw and drain sink . If no water travels to the dishwasher the issue is with dw not getting rid of waste water correctly.
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If water is traveling from sink to dish washer there may be a change in how your sink is draining like a partial blockage in drain line or an obstruction in plumbing vent.
Edit Since you commented that you tried the experiment above but saw no water in dishwasher and sink drains well the only other thing I can think of is perhaps it is filling with fresh water. A faulty inlet valve could cause this. Even fresh water will get gross if it sets in the bottom with the traces of bacteria laden gunk in the strainer etc... Try turning supply off for a week to test this theory.