Typically there is a plastic tube going to a diaphragm pressure switch
somewhere near the top, inside the machine.
Do a Google image search on "washing machine diaphragm switch" to see
what they look like - although the easiest thing to spot is often the
clear plastic tube rising up through the inside of the machine.
The switch could have failed - or the tube could be blocked.
Be careful - metal, mains and moisture are a pretty lethal combination.
Most dishwashers that I have worked on have the sensor, (pressure
switch) in the door panel. As Palindrome says, the plastic tube to the
pressure switch could be blocked, or the connector that the tube fits on
at the bottom of the machine could be blocked. These pressure switches
are usually reliable, unless the machine has been laid on its side and
water has gotten into it.
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