Loose belts would cause a printer to malfunction and prevent proper printing. It can cause Layer shifting, Ghosting, or other print abnormalities like getting an irregular shape instead of a perfect circle, when printing a cylinder. The Y-axis belt is located under the heatbed, X-axis belt moves the print head. All adjustments are done with the 2.5mm Allen key.
Run the Self-test or the Belt test, from Calibration on the LCD. The check LCD-menu -> Support -> Belt-status. You want the number between 260 and 290 (275~). The lower the number, the tighter the belt is. Imagine it is a fictive count of teeth in circulation.
Another way is to use the technique described in this step of the assembly manual to test if the pulley is correctly tightened and if the belt is not too loose. Hold the X-axis motor shaft with pliers (take advantage of the flat part of the shaft) to fix it in place (purple circle), then try to move the extruder by hand. You should see no slack on the belt as a result of pushing on the extruder. The same procedure can be applied to test the Y-axis pulley and belt.
Loosen the two front-facing screws and make sure they have space to their right to move (yellow circles). While assisting with the rotation of the motor with your hand, tighten the M3x18 screw on top of the X-end-motor (purple arrow).
After each turn or two check the tension in the belt by pressing them together. For optimal performance, the belt must need some force to press together with your fingers (green arrows). Move the extruder all the way to the X-end-idler and try the belt tension in the middle of the X-axis.
If a slight adjustment is not sufficient you must adjust it at the belt-holder. This is found on the back of the extruder and requires a little disassembly.
First, remove the two top screws from the motor-holder to let the belt have some slack in order to easier manipulate the belt.
On the front plate of the printer's frame, behind the LCD module, you will find the printed Y-axis belt idler. This is held by two M3X10 screws (purple arrow), going through the front plate.
If you have a gap between the idler part and the front plate of the frame (blue arrows) you can tighten the belt by using your 2.5 mm Allen key to turn the screws clockwise (purple arrow), closing the gap.
The easiest way to access the screws is from below, by moving the printer to the edge of its table, with the LCD module sticking out. You now have access to the bolts from below the LCD module.
If a slight adjustment is not sufficient you must adjust it at the belt-holder. There are two main types of the Y-axis belt holder, one adjustable and one fixed. The adjustable belt holder was introduced in the first quarter of 2019, with the S-upgrade for the Original Prusa MK3. Adjustments for this are explained in 'Method 1'. For an earlier Original Prusa MK3, follow 'Method 2', for the fixed belt holder.
Your belt mount may look slightly different, being another iteration, but the procedure remains the same.