You notice that perhaps your bed, or nozzle is not heating up. This page will help you troubleshoot this problem.
If the LCD temperature is -15C for either the bed or nozzle, then either the connection going from the sensor to the motherboard is broken, or the sensor itself is broken. Check all the connections are plugged in FIRMLY! Examine the connector plugs also, check that no wires are loose or coming out of the connectors. If the LCD temperature shown is 0C, then there may be a temperature sensor connection problem, or, there may be a communication problem between the LCD and the main motherboard, so you need to check the LCD connections as well. If the temperature is around 300C, then you may have a short circuit, or a damaged port on the motherboard - see this page for further advice.
Download the free software Pronterface (also known as Printrun). With this software, you can connect to your printer from a computer, using a USB cable, and even print gcode files over the cable. This software will show you the raw log out of the printer, which is where any major errors are reported. You can then search the error you see in google, or ask on the forum, or on one of the JGAurora Support groups.
After installing Pronterface, connect to the printer using Baud 250000, and choose the correct serial port for your printer.
If you can't connect to any serial port, you may need to install drivers for the CH340 USB chip. To find your serial port (also known as COM port), unplug the printer, and see which com ports are currently listed. Plug the printer USB cable in, and see if any new COM ports appear. Those will likely be your printer. If you can’t find the right com port, or your device is not detected, you may need to install drivers for the CH340 serial to USB interface chip in the printer. Mac CH340 serial drivers are here. PC CH340 serial drivers are here. On a mac running high-sierra, I had problems with those mac drivers, and I had a more reliable connection with the mac drivers available for purchase (unfortunately… not free, but someone gotta eat) here.
If you connect to your printer via Pronterface you will hopefully see an error message that will help you troubleshoot the problem.
If you see an error about “MIN_TEMP” the printer has detected a temperature that is too low, and it thinks that a sensor is either damaged or missing. For safety, the printer will then refuse to heat up, to prevent burning your house down! There is a broken wire, or loose connection somewhere. Perhaps a leg of the thermistor has broken off, or a cable is detached or loose?
If you see an error about “HEATING FAILED” this means that the printer did not detect a sufficient change in temperature while heating up. This is a safety feature, and suggests there is a fault with wiring somewhere - energy is going in, but it is not ending up in the bed. Perhaps there is a loose connection somewhere - for example the heated bed connector may be damaged.
For more info on the thermal safety systems in the Marlin Firmware, see this page.
If you know how to use a multimeter (or you have a kind friend who does), you can measure the resistance of the thermistors (temperature sensor), and they should measure approximately 100K ohms at 25C. From there, you can trace the cables, plugs, and wires until you find the broken connection. You can check the resistance of the heated bed or nozzle heater cartridge as well, the bed should measure around 2 to 3 ohms, and the nozzle should measure around 15 to 20 ohms. Check the resistance at the sensor/heater directly, and at the motherboard connector as well. A different measurement at the motherboard indicates a cable problem.
A video tutorial on replacing the Hot-End Thermistor can be found here.
If you are still getting faulty temperature readings after checking your thermistor temperature sensors are ok, it may be that your motherboard thermistor sensor port is damaged. There is information on using the spare port here.