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So you are correct that F11 is a drain error. You've already checked the non-return valve and the pump veins rotating forward and backwards with sort of a resistance type click is normal. This is caused by the magnets on the motor. Either way, you will indeed need to pull the dishwasher out and then remove the bottom access panel below the dishwasher. Disconnect the drain hose from below the dishwasher tub and blow through it to make sure it's clears. If not then try to locate the clog, otherwise replace the drain pump.
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Hello again. I'm just following up because I haven't heard back from you and I see this question is still open. Is there anything else I can help you with?