1. Restore factory settings>>.  
2. Check the Z-axis mechanical components, including the belt tension, the threaded rod, mechanical connections, and the workbed, for signs of unevenness, looseness, or breakage. Replace with identical ones as needed. If any, replace them with identical ones.

If all those have failed,

1. Open the access door.
2. Check if the indicator of the Z-axis driver is glowing.

If it’s not, see "Driver not lighting up>>" to troubleshoot.

If the indicator is glowing, check if the input voltage of the driver is normal (24V or 36V).

If it is not, see "Abnormal voltage of switch output>>" to troubleshoot.

If it is, normally measuring 24V o6 36 V,

• Swap the motor plug of the driver glowing red with that of the one glowing green (as shown).

• Check if the Z-axis driver is glowing green and is moving the axis with no noise.

If not, the Z axis is still malfunctioning, then its motor is faulty; replace it with a new, identical one.

If so, the Z-axis is working fine,

• Swap the remaining plugs on the two drivers.
• Check if the Z axis is malfunctioning or not.

If so, the mainboard is faulty; replace it with a new, identical one.

If not, then the Z-axis driver is faulty, replace it with a new, identical one.