Have you noticed the gantry becomes crooked during Y axis movements? It's possible that you just need to manually align them.
With the printer unplugged, take the horizontal cross bar and slide it to the middle of the printer.
Now, firmly grasp one slider to hold into place and slide the other slider towards the direction of the other to align them Note: some force is required to overcome the friction
You'll hear a little clicking as the slider moves
You can slide the horizontal cross bar to the front of the printer to check that the sliders are aligned similarly to the photo below
If the sliders become unaligned again or move independently of one another, it is likely a belt may have broken or jumped off pulley teeth. You can check this by running the Y skip test in the spooler (Settings Icon>>>Advanced>>>Expert Mode>>>Diagnostics tab) and see if there is any movement from the right side gantry or if it is just being dragged by the left side gantry. If the Y belt inside the base area is ok then the left or right gantry, which lost it's belt should probably be replaced.
It is good to know that these two Y belts on the left and right carbon rods are not manufactured as a continuous loop.
Second, the gantry can become crooked during a print job if the small belt inside the base area has been detached from the pulley teeth. There is a belt inside the base (Y belt) that connects the left side gantry Y motor to the right side gantry. As you can see in the image below, the smaller Y belt on the motor, which is located underneath the base, is a continuous loop. The Y belt on the motor is most likely the belt that needs to be re-latched onto the pulley teeth.
To fix the Y belt inside the base, please follow the instruction below:
Pull on the box top, while putting your hand on the base. The box top will click and slide off the tabs that it is connected to. Be aware of any bearings or tiny washers that fall out during this process, because you will need to place these back on during re-assembly.
Once you have slightly popped the box top upward, you can peek under to see if the belt has been detached from the pulley teeth. It is not necessary to completely detach the box top from the printer.
If so, then use a small hook tool to pull the belt back onto the pulley teeth.
Once you have successfully latched the belt onto the pulley, please move the print head back and forth to ensure the gantry does not become crooked during movement.
Next, continue to re-mate the box top onto the base, while ensuring the bearings and washers are in their correct positions as before.
Once all parts are in the correct positions, then you can slide the steel rods into the bearing holes on the top side of the box top.