Print square wall but output rectangle wall

Hi all,
This is my first time use Reprap forum.
I built 3D printer half year ago, I met an issue several times that when I print square wall but output rectangle wall. Y axis is shortened. Before it was recovered automatically, so I didn’t pay attention to, but recently it output rectangle all the time. I checked the Gcode, it is correct to draw a square. I manually controlled Y axis back and forth for 100mm, it is OK.
I tried different version Repetier but has same result.
I don’t know what the problem is
Thanks
James 4D

Here is picture

Can you show the gcode file you used, maybe there is some clue in there as to what is going wrong?

Looking at the picture, it seems to be the X axis that's printing short? Unless you have spun them 90 degs since removing them.

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Waitaki 3D Printer

Maybe your steps per mm has changed in your firmware? ( on the long or short side of the square)

are your x and y axis gears the same diameter? do you know what your firmware setting is for x and y steps?

It could also be that the pot on your y motor driver is turned too low and you are skipping a few steps each time, though this seems unlikely given that you have a perfect rectangle.

As the people above have mentioned: be sure to check your firmware, g-code, belts, and pulleys,

Eric