3D Printers

General Principle of Operation
There are a few different design philosophies for 3D printers, but many of them share the same roots. Debugging and diagnosing printers can be relatively straightforward once you know how all the parts interconnect. The general working principle is a positioning system with limit feedback, a heating element with temperature sensors and extrusion mechanism, a power supply, and a control board running firmware.

Positioning System
There a few a different positioning systems commonly used on 3D printers. This includes Cartesian (XYZ), Delta, and CoreXY. Each of them has their positives and negatives, but outside of this, debugging them is fairly straightforward. If you think you have an issue with the positioning or extrusion motors, you can test this easily by feeding the printer the G code for the axis you believe is faulty. You can find this information easily by reading the reference for your firmware. For example, here is the gcode reference for printers running Marlin. Just direct one of the axes to move back and forth and see if it works. This also works for the extruder as long as there is no filament loaded.

Heating Element/Hotbed
If you suspect the heating element is not working, you are likely right. It's very obvious to tell. However, to guarantee that you're telling the printer that you want the temperature raised, you should reference the gcode and directly send the command yourself (there could be unexpected behavior inserted by your slicer during gcode generation that gets in the way of heating; you never know).

Control Board
The control board is usually a pretty simple microcontroller, but have become a bit more advanced recently. The low end of these boards usually run an ATMega 8-bit microcontroller, but now you can get them running full 32-bit ARM microcontrollers. This also houses the driving amps for heating the hotend and bed, as well as the stepper drivers for controlling the positioning system.

Power Supply
These are typically just Meanwell generic Chinesium power supplies. Depending on your printer, you may have a 12V system or a 24V system. 24V systems are generally used for printers that have larger motors and better heating elements. Doubling the voltage means halving the current needed, leading to cooler and longer lifespan for the control board. These are easy to replace as long as you can find one with the connector you need to pair it with your control board.

Extruders
This is the component that pushes the filament from the spool and out of the nozzle. There are two main categories of extruder, these being bowden and direct drive, both coming with their pros and cons.

There are many different types of extruder that fall into the two categories; most extruders can actually fall in both, depending on their installation. The most common ones include but aren't limited to: MK8 (no gear reduction, single drive), Titan (3:1 gear reduction, single drive), BMG (3:1 gear reduction, dual drive).

Nozzles
Available in different materials and sizes, nozzles melt and deposit the filament to the print bed (and subsequent layers on top). They are one of the many parts that make up a hotend assembly, directly in contact with the heater block and the heatbreak.

The standard brass nozzles are incredibly cheap and easy to replace (we're talking $2 for a pack of 10 shipped), so in the event of a clog you'd be better off getting a new one.

Note: Some material like Glow-in-the-dark filaments, carbon fibers filaments, wood filaments and other exotic filaments can cause extremely fast wear of brass nozzles. If you are using these types of filament, a stainless steel nozzle could be preferred.

Printbed
Also referred to as build plate, this is where the first layer gets laid down. It's important to have a well maintained bed, allowing the plastic to stick when it's supposed to and get released after the print is competed.

The most common print beds in cheap consumer printers are made of glass, either plain or textured. Glass is hard, flat and consistent: this helps keeping the cost down, as the heated bed below it doesn't require tight tolerances as it simply conforms to the glass. It also has no issues with adhering to the most common materials, assuming it's a clean surface.

Cleaning glass is very simple: a low amount of high concentration isopropyl alcohol will wipe away any oils that might impact adhesion.

Another common material to print on is PEI, as its sheets can be applied to any surface and, like glass, can come in plain or textured. PEI sheets are usually combined with a magnetic spring steel plate which allows for convenient and simple removal of prints, by flexing the plate and literally popping them off.

3D Printer Model Specific Information
Lulzbot Taz / Lulzbot Mini

Creality CR-10

Anycubic Kossel Linear Plus

Prusa i3 mk2s