In 99,9% of the cases where an axis (X, Y or Z) moves only in one direction the cause is the end stop (regardless of whether mechanical or optical).
Maybe it needs to be inverted, maybe it isn't working.
Most firmwares allow deactivating the end stops so you might try this first.
Basically the microprocessor is reading the signal pin for the Y MIN end stop and it says that it has reached the minimum position so the firmware does not allow any more travel in this direction.
Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
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