3D Printer Hotend Temperature Fluctuations – Causes & Fixes

As hotend temperature is one of the primary factors that directly affect the rate of filament flowing out of your 3D printer’s nozzle, it’s vital for your 3D printer to maintain the target hotend temperature in a stable manner without any fluctuations involved for a successful print where over-extrusion or under-extrusion does not occur.

In this guide, we will discuss the factors that can cause your 3D printer’s hotend to have temperature fluctuation problems and go through the solutions that will help you resolve this problem to restore stability to the hotend temperature once again for a consistent print without inconsistencies in extrusion.

3D Printer Hotend Temperature Fluctuations – Causes & Fixes

As it’s possible for the hotend temperature fluctuations to stem from both hardware and software-related factors, including some false positives due to issues related to temperature readings rather than the actual temperature of the hotend itself, there are a fair few factors to go through to find out which one the root cause is.

Incorrectly Tuned PID / MPC Values

We can say that incorrectly tuned PID (or MPC) values are the leading cause behind the majority of cases where a 3D printer undergoes hotend temperature fluctuations, as these values are essential for the firmware to reach the target temperatures as optimally as possible and keep them stable throughout the print.

For those unfamiliar, the term PID refers to three values known as Proportional, Integral, and Derivative, which we can effectively describe as a system that makes it possible for your 3D printer to calculate the correction it has to apply to the current temperature of the hotend (and the bed) to make it reach the target temperature as quickly as possible and without any fluctuations.

While less common compared to PID, it’s worth mentioning there’s another system that serves the same purpose of correcting the hotend temperature effectively, known as MPC (Model Predictive Control), which you can also come across as a replacement for PID depending on the firmware your 3D printer has installed (as an example, Marlin Firmware can either use PID or MPC based on how it’s configured).

ender 3 v2 neo mpc tuning example

Regardless of whether your 3D printer is using MPC or PID for running the temperature calculations, in cases where these values aren’t configured correctly for the target temperature you usually heat your hotend to for a print, your 3D printer’s firmware won’t be able to calculate the corrections it needs to apply to the current temperature of the hotend in an optimal way, which will create a scenario where the hotend temperature keeps undershooting or overshooting and fluctuates as a result.

Fortunately, resolving this problem is pretty straightforward, as regardless of the 3D printer or firmware you’re using, you should have access to an automatic PID tuning feature, both through G-code commands and the menus of your 3D printer, where you enter the target temperature to tune the values for, and the firmware does the rest automatically for you without requiring further manual input.

As an example, if you’re using Marlin Firmware in PID control mode, you can use the M303 (PID autotune) G-code command or the related menu entry for the PID autotuning process together with the target temperature of your choice to tune the values for and save the updated PID values to the EEPROM with M500 (Save Settings).

On the other hand, on a 3D printer that runs Marlin Firmware in MPC mode, you would need to use the M306 (Model Predictive Temperature Control) G-code command (or the related menu entry, which would be named MPC tuning instead of PID tuning) instead to tune the MPC values of your 3D printer, and once again, save the updated MPC values to the EEPROM with M500 afterward.

As a rule of thumb, we highly recommend tuning your PID (or MPC) values whenever you’re setting your 3D printer up for the first time, making a hardware change to the hotend, whether it’s replacing the heater cartridge or installing a new heat block, switching to a new type of filament which you will be printing at a different temperature, or flashing new firmware, which will ensure that the values are up-to-date for optimal heating.

Misplaced / Loose Heater Cartridge

Another commonly encountered issue that can lead to fluctuations in the hotend temperature is the heater cartridge coming loose, which is especially likely to be the problem with unassembled or partially assembled 3D printers such as the Ender 3 after setting it up for the first time, or after making modifications to the hotend.

When the heater cartridge comes loose from its spot, it will both lose its ability to provide the necessary heat in an efficient manner and also end up constantly moving during the printing process, which will naturally lead to the hotend temperature fluctuating due to the contact area between the heater cartridge and the heat block changing throughout the print.

While the exact process will be different for each 3D printer, our primary recommendation, in this case, would be to disassemble the hotend and ensure that the heater cartridge sits tightly in its slot on the heat block without any wiggle by trying to get it loose with your hand or to refit the heater cartridge into its correct place from scratch if you are unsure whether it’s tight enough, which will allow you to rule this possibility out.

Misplaced / Loose Thermistor

Similar to a loose or misplaced heater cartridge, a misplaced or loose thermistor can also be the reason behind the temperature fluctuations you’re experiencing during a print (but in a different way), which once again is especially likely to be the source of the problem in cases where you’ve set your unassembled or partially assembled 3D printer up for the first time, or made modifications to the hotend recently.

In a scenario where the thermistor ends up coming loose from its spot, it will lose its contact with the heat block throughout the print as the printhead is moving from one side to the other, which will create a scenario where the temperature readings sent to the firmware end up fluctuating due to the thermistor not being able to take stable temperature readings with complete contact to the heat block at all times.

As the firmware will react to the changes in the temperature readings in an attempt to keep the hotend temperature stable, even when the actual temperature provided by the heater cartridge remains the same (which the firmware won’t be aware of), the instability in the temperature readings will lead to fluctuations in the actual temperature of the hotend despite the thermistor itself not being responsible for providing the heat.

Similar to the case of a loose heater cartridge, our recommendation, in this case, would be to disassemble the hotend and ensure that the thermistor is positioned correctly within the heat block by referring to the manuals and videos produced by your 3D printer’s manufacturer, which will make it possible to verify that the thermistor is indeed installed the way it should be.

Damaged / Loose Heater Cartridge Wiring / Connector

Wiring problems regarding the heater cartridge is another possible culprit that can lead to your 3D printer’s hotend having temperature fluctuations, which, once again, are a whole lot more likely to occur in scenarios where you have recently interacted with the mainboard and the hotend or set up your 3D printer for the first time.

As an issue with either the wiring or the connector will compromise the connection between the mainboard and the heater cartridge, your 3D printer won’t be able to regulate the temperature correctly, which can lead to numerous issues regarding the hotend temperature, including fluctuations you’re experiencing.

In this case, our primary recommendation would be to unplug the heater cartridge from the mainboard and to carefully examine the cable for any signs that may indicate there is a problem with it, whether it’s exposed wires, severe bends that can lead to the wires inside being broken, burn marks, connector coming off the cable, or anything else that comes to mind pointing toward a compromised cable.

Provided that the wire seems intact, our following recommendation would be to ensure that you seat the connector into the corresponding pins on the mainboard correctly, which, under normal circumstances, should sit tightly without any wiggle that could lead to it losing contact with the mainboard while your 3D printer is operating.

Damaged / Loose Thermistor Wiring / Connector

Similar to how wiring issues related to heater cartridge can cause fluctuations, wiring issues related to the thermistor can also contribute to the occurrence of temperature fluctuations, which, yet again, are a whole lot more likely to be present in cases where you have set your 3D printer up for the first time or made hardware modifications.

When the wire that provides the connection between the thermistor and the mainboard is compromised, the temperature data that the thermistor sends to the mainboard will naturally end up becoming flawed, which will create a scenario where the mainboard doesn’t have access to correct data regarding the current temperature of the hotend, which, in some cases, can present itself in the form of a fluctuating temperature value.

In such a scenario, our first recommendation would be to remove the connection between the mainboard and the thermistor and examine the thermistor cable for any signs that could potentially mean it’s damaged, whether it’s exposed wires, bends that can break the wires inside, burn marks, the connector and the cable separating, or anything else that comes to mind, similarly how you have inspected the heater cartridge wiring.

If there don’t seem to be any issues with the wire itself, our following recommendation is to ensure that the thermistor is connected to the mainboard correctly, with the connector sitting tightly in its spot without any wiggle, whether it’s a connector that you plug into the pins on the mainboard, or one that you screw to tighten.

Damaged / Defective Thermistor

Considering that there are no issues regarding either the wiring of the thermistor & heater cartridge or the PID tuning, an issue regarding the thermistor itself becomes something to consider when experiencing hotend temperature fluctuations, especially considering that it’s a component that can eventually wear out due to usage.

As it’s possible to come across numerous temperature-related issues when printing with a defective thermistor, whether it’s entirely incorrect temperature readings or no temperature readings at all, fluctuating temperature values can definitely be a sign that the thermistor is not operating as intended, which makes it necessary to rule this possibility out when troubleshooting this problem.

Considering that it wouldn’t exactly be possible to repair the thermistor, our recommendation in the case where you have gone through everything else in the list so far without solving the fluctuation issue you’re experiencing would be to replace the thermistor with a new one, which will allow you to check a thermistor-related problem off your list in a straightforward way that won’t take up too much of your time.

Damaged / Defective Heater Cartridge

Last but not least, a damaged heater cartridge can be the culprit behind the temperature fluctuations you’re experiencing, and even though coming across a failing heater cartridge is less common than coming across a failing thermistor, it’s still something to consider if you haven’t been able to find the cause of the problem anywhere else.

While how exactly the heater cartridge behaves, whether it’s not heating up at all, overheating, or not being able to hold the temperature and cause fluctuations, as a result, will depend on the type of damage that affects it, similar to the case of a problematic thermistor, a damaged or defective heater cartridge is definitely capable of creating the hotend temperature fluctuations you’re experiencing.

Regardless, our primary recommendation in the case where you suspect that the heater cartridge is damaged or defective would be to replace it with a new one, especially considering that it’s not an expensive component to purchase by any means, as this will practically allow you to rule out any issues regarding the heater cartridge itself and check this possibility off your list when troubleshooting the hotend temperature fluctuations you’re experiencing.


Having your hotend temperature fluctuate during printing is one of the more urgent issues that you should be looking to fix as soon as possible due to the considerable negative effect it will have on your prints, even if it may be challenging to find the root cause and apply the correct solution due to the likelihood of the issue stemming from a hardware-related problem, which is usually more demanding than dealing with than software.

That being said, even though the list of factors that can cause this problem is pretty long with a lot of hardware-related issues involved, tuning the PID, which the firmware can do automatically, and replacing both the thermistor and the heater cartridge should almost always guarantee that the fluctuations are gone, meaning that this is not an impossible problem to resolve by any means.