The screen on my printer is showing a “Thermal Runaway Error”, how do I fix this issue?

This may be caused by many issues.
1. It could be a worn-out or broken heater cartridge.
2. Broken wire to the thermistor
3. Heater block is too close to the heatsink

To diagnose any of these issues, you will need a multimeter. You may need to take the hotend and/or control panel apart so have access to some tools. Let the machine cool to room temperature before starting diagnoses. First, let’s check the resistance on some parts. Make sure everything is unplugged from wall-power. To check the heater cartridge, you will remove the access panel to control board, unfasten terminals that hold the heater cartridge wires, remove these wires, and use the multimeter (Ohms setting) and place each lead of the heater cartridge wire to multimeter probe leads. On a 12V 40W printer, it should read 3 Ohms. On a 24V 40W printer, it should read 14 Ohms. If it reads “infinite resistance” then you will need to replace the heater cartridge and/or wires. This may be the cause of the issue.

To test the thermistor, remove the thermistors JST connector from the control board, place multimeter leads across the terminals of the thermistor JST connector. The readings should be as follows:
- For NTC100K = 100,000 Ohms
- For NTC3950 = 100,000 Ohms
- For PT100K = 100,000 Ohms
- For PT100 = 100 Ohms
- For PT1000 = 1,000 Ohms

If any of the readings read “infinite resistance” than the thermistor will need to be changed and is
possibly the cause of the issue. If it reads 0 resistance, there is a short from your wires to ground,
and you will need to replace the thermistor.

To see if the heater block is the cause of this issue, then you will need to take the hotend apart.
When you are looking at the hotend, make sure the distance between the heater block and heatsink
are 2-5mm apart (depending on machine). Creality machines are usually 2mm apart.

It should be pointed out that the readings for the thermistor are approximate and are valid if measured at, typically, 25C. If you’re measuring it in a warm room or a cold garage, you can get wildly different values as these devices tend to be non-linear.

The main point, of course, is that you shouldn’t measure infinity and you shouldn’t measure 0. Just don’t think that you have a problem because you measured your 10K thermistor as 19K in your 10C garage.

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