Our XYZ DaVinci Junior 3D printer wouldn't work, it kept making a loud juddering noise and showed error code 0030. Here's how I fixed it.
This step is difficult, turning on the printer causes the hotend to move too far to the left, and if the Z height is too high it's impossible to reach the nut holding the boden tube on. Another issue is that the printer won't actually work, so you can't heat the hotend up to unload the filament.
Manually move the hotend to the side and carefully use a screwdriver to press the hotend release button. Once the hotend is off, you can unscrew the boden tube and use a pair of cutters to cut the filament.
The door is attached with four screws on its hinge. Put them back in the holes so they don't get mixed up with other screws later. There are no wires attached to the door so it comes off easily.
The top can be removed by undoing two screws on the back. Slide the top towards the back of the printer and it will lift off. Be careful of the boden tube, it will need unthreading from the holes in the top.
The side panels are tricky to remove. There are two machine screws with washers on the top, and then four plastic clips on the bottom. Undo the screws first and then replace in the holes they came out of. Use a screwdriver to press the tabs on the bottom of the printer, starting at one side. There is no need to use prying or excessive force, the clips are very easy to press. If you're prying the case, you're doing it wrong and have probably missed the two large plastic tabs in the middle.
Don't forget to unplug the printer too. The power cord goes through a side panel.
The back panel is held on with four machine screws that go into the aluminium channel. Inside each channel are two nuts, they will fall down the channel as you remove the bolts, but can't come out completely.
Unplug the fan from its cable, and store the four bolts in the case.
Manually turn the threaded rod with your fingers to lower the Z axis. It needs lowering so that you can easily see what is going on from the back of the printer. The threaded rod is covered in grease, so turn the coupling between the motor and rod instead of the rod itself.
The hotend's optical sensor has a bar that moves between the two parts of the sensor. Manually move the hotend carriage to check this is not obstructed. At this point you can turn on the printer and see if it homes correctly. The problem with our printer was mechanical, the small plastic arm wasn't moving into the sensor correctly. If that is OK on your printer but it doesn't work still, the sensor itself might be dirty or faulty. It works by shining infra-red light onto a sensor, so if either are dusty or dirty they won't work.
The sensor on the other axes are the same and can be swapped to test if the sensor or its cable are faulty.