If a washing machine is overfilling with water you should be able to see that the water line inside the drum has gone much higher than normal or has gone completely up to the top of the door glass. Eventually water will start pouring out of the soap dispenser drawer and / or out of the back.
However, as most modern washing machines can detect if they are overfilling and abort the wash cycle you may not even realise what’s happening. You may just notice that the washing machine starts to fill but never stops and then aborts and starts pumping out the water.
If the washing machine is overflowing with foam or soap suds try this article – too much foam in the washing machine?
Troubleshooting an overfilling washing machine
If the washing machine is overfilling but water continues to enter when you switch it off then either the hot or the cold water valve must be jammed open. This can be caused by debris from the water supply getting past the water filter, or just a fault inside. It’s rare though. In this situation the water must be turned off at the taps and a new water valve fitted.
If it does stop filling as soon as you switch the washing machine off then the fault is somewhere within the pressure system that controls the water levels.
An overfilling washing machine is most commonly caused by a blockage of grease and gunge in the pressure chamber at the bottom of the drum. A blockage prevents air pressure rising up the thin hose connected at the top of it.
But there are other possible causes and complications. I can’t give specific repair instructions. As with many faults, you need experience and technical knowledge to successfully repair it. However, if you at least fully understand how the water levels are controlled you may be able to work out what’s causing the problem.. (read on)
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Before linking to more information about how the washing machine controls water levels here are a few similar faults that may be relevant
Washing machine fills but empties down the drain at same time
If water is running down the drain (or being pumped out of the machine) but it’s filling at the same time it could be caused by a fault described here – Washing machine fills and drains at the same time
Washing machine fills with water when off
If water runs into the washing machine when it isn’t even running try this article Washing machine fills with water when off