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Whirlpool uses a flow meter that detects the flow of water entering the washer. If this flow meter does not detect the correct amount of water entering the washer it will sound the F20 error code. Often this repair can be made with out calling an appliance
repairman and without ordering parts. Start by removing the washer fill hoses; both the cold and the hot water hoses. Check for proper water pressure from the house. To do so, turn the water on and put the hose inside the washer or a bucket.
If water pressure is good, check the water valve inlet screens. Most residential washing machines have inlet screens. The screens prevent minerals from clogging the solenoid. With the hoses removed, you can visually inspect the water inlet screens. If the water valve screens are clear the inlet valve with the screens itself will need to be replaced. To replace the water valve, remove the top of the washing machine. Three screws hold the top down. The screws that hold the top down are on the back of the top panel. Once removed the top will slide back and off. Now remove the wire harness that connects to the valve. One screw holds the valve to the washers cabinet. Removing the screw and sliding the valve over will release the valve from the back panel enabling you to complete your repair.
One last thing to check is the fill hoses. These front load washers call for a lot of water quickly. So if you have stainless steel braided flood-safe hoses, they will trip for no reason at all. Replace these flood safe hoses with more traditional hoses. If the hoses are good and you haven't found anything slowing down the flow of the water, your electronic inlet water valve is just faulty on the back of your washer and will need to be replaced because it's just not allowing water flow into the washer correctly.