Ender 3 / Ender 5 Thermal Runaway Error – Causes & Fixes

A thermal runaway error, which essentially means that the thermal runaway protection feature has kicked in and prompted the firmware to halt your 3D printer due to inconsistencies regarding temperature readings, is one of the more critical problems you can face when 3D printing, as an undetected thermal runaway can easily lead to a catastrophic event such as a fire.

In this guide, we will discuss the factors that can cause your Ender 3 / Ender 5 to display a thermal runaway error on the LCD controller and take you through the solutions that will help you resolve this problem as quickly and conveniently as possible.

Ender 3 (Pro & V2 & S1 & Neo) / Ender 5 (Pro & Plus & S1) Thermal Runaway Error – Causes & Fixes

Since the thermal runaway protection feature in Marlin firmware activates in any case where the temperature readings from the sensors aren’t in line with the target temperature after the pre-heating process has concluded and the print has started (even if a thermal runaway isn’t actually about to take place), we can say that there are a fair few culprits to put into consideration when trying to find out what’s causing the thermal runaway error in any given scenario, which we have listed below.

marlin thermal runaway protection explanation from source code


While not always a part of the error messages depending on the firmware you’re using, the appearance of the text E1 or Bed on the screen with the thermal runaway error (such as THERMAL RUNAWAY: E1 or THERMAL RUNAWAY: Bed) would tell us that that the thermal runaway issue is hotend or print bed related, respectively, which is something to keep in mind when diagnosing the error.

ender 3 ender 5 thermal runway error example


Incorrect PID Tuning

PID tuning refers to the process of optimizing the Proportional, Integral, and Derivative values your Ender 3 / Ender 5 requires to reach the target heated bed and hotend temperatures of your choice optimally and keep them stable throughout the printing process.

In a case where these values are not optimal, which can be a result of various factors such as not PID tuning your 3D printer at all, switching to a type of filament with substantially different temperature requirements and not PID tuning afterward, or replacing the hotend or the heated bed of your 3D printer without PID tuning, your printer will have a more difficult time keeping the temperature stable, which will present itself in the form of temperature fluctuations & overshooting, and possibly lead to the thermal runaway protection kicking in as a result of the inconsistencies.

Fortunately, resolving this issue is pretty straightforward, as your Ender 3 / Ender 5 can automatically tune both the hotend and heated bed PID values for the target temperature of your choice with the M303 (PID Autotune) G-code command, which you should also be able to find in the menus of the LCD controller, such as in the example image below.

ender 3 v2 bed pid tuning example


As the menus can be different for each firmware, we will quickly be covering the process of automatically tuning the PID values of your Ender 3 / Ender 5 with G-code commands for the purposes of this guide to make things more straightforward, which should work for any 3D printer running on Marlin Firmware (unless your firmware is configured to use MPC instead of PID, which will require you to refer to the M306 G-code command documentation instead).

To use the M303 G-code, all you will need to do is to include the cycle count (more cycles, more accuracy) with the C parameter, the hotend index with the E parameter (-1 for heated bed), and the target temperature you would like to use with the S parameter, which will initiate the PID autotuning process.

So, for instance, to PID tune the hotend of your Ender 3 / Ender 5 for a target temperature of 220 degrees Celsius with a cycle count of 10 (which is what we recommend using), you will only need to send the command M303 E0 C10 S220, and the firmware will do the rest for you.

On the other hand, to PID tune the heated bed of your Ender 3 / Ender 5 for a target temperature of 60 degrees Celsius with a cycle count of 10, you would be sending the command M303 E-1 C10 S60 instead, which once again will get the firmware to handle the entire process.

m303 pid tuning bed example


Finally, when your 3D printer finishes up the PID tuning process, you will need to save the changes to the EEPROM by either using the M500 G-code command or the corresponding menu entry (Store Settings), which will ensure that you don’t come across issues related to the PID values anymore.

Thermistor Detached from Hotend During Print

A thermistor is a component located in both the hotend and the heated bed of an Ender 3 or Ender 5, responsible for reading the temperature and reporting it to the firmware.

In the case where a thermistor detaches from its correct place as the print is taking place, where both the hotend and bed temperature should stay stable at the target value throughout the entirety of the process under normal circumstances, the temperature readings of the firmware will instead start plummeting (or fluctuating if the thermistor becomes loose but not entirely detached) as a result of the thermistor effectively measuring the ambient temperature around it rather than the temperature of the hotend or the heated bed.

As the thermal runaway protection feature constantly checks for the difference between the current temperature and the target temperature and halts the print in cases where the target temperature stays larger than the current temperature (determined by the hysteresis value) for longer than expected (determined by the period value), a detached thermistor immediately leads to a thermal runaway error.

Even though the placement of a thermistor is unlikely to shift on its own under normal circumstances, as it’s supposed to be tightly secured into its place both in the case of the heated bed and the hotend, meaning that thermistor detaching is even more likely to be the cause of the issue if you have recently set your Ender 3 / Ender 5 up for the first time, disassembled the hotend, or disassembled the heated bed, the fact that it creates the majority of the thermal runaway errors makes it the first thing we recommend checking regardless.

In the case of the hotend, you can find the thermistor on the side of the heating block, where it’s attached to the heating block with a standard screw that locks the thermistor wire in place for the thermistor to go into its slot (you can refer to the video below to see exactly where it is).

If the thermistor is correctly installed, it should be seated all the way into the corresponding hole, with the screw holding the wires in place to completely prevent the thermistor from moving, and if this is also what you’re seeing on the hotend of your Ender 3 / Ender 5, we can practically say that a detached thermistor isn’t what’s causing the thermal runaway error for you.

On the other hand, in the scenario where the thermistor seems to be somewhat loose when you tug on the thermistor wire; or you’re unsure whether it’s secured correctly, our recommendation would be to remove the screw, re-attach the thermistor in a way where it’s entirely inside the hole, and re-tighten the screw again very carefully, as the screw should only be tight enough to keep the thermistor in place, but not so much that it ends up damaging the wire, which will allow you to rule out issues regarding thermistor placement completely.


When it comes to the heated bed thermistor, you will find a cotton heat insulation pad taped on the underside of the heated bed, and underneath it, another layer of tape that secures the thermistor to the heated bed without any complex mechanisms involved (you can refer to the video below to see exactly where it is).

Even though it’s close to impossible for the bed thermistor to become detached, as there’s only one straightforward way to attach it, which is to tape it onto the bed with practically no wiggle room, there’s no harm in ensuring that everything is in order either, especially considering that you should be able to see the underside of the heated bed without disassembling anything and quickly find out whether it’s coming loose or not.


While the above video is for a Creality CR-10, how the heated bed is structured is practically the same for an Ender 3 / Ender 5.

Heater Cartridge Detached from Hotend During Print

The heater cartridge (heating tube) is a component located in the hotend of your Ender 3 / Ender 5, responsible for supplying the heat that allows the hotend to melt the filament as the 3D printing process takes place.

In a scenario where the heating cartridge detaches from its allocated position within the hotend assembly during the print, where the hotend temperature should be stable throughout under normal circumstances, the temperature readings will instead undergo a sudden drop (or fluctuations if the heater cartridge is loose but not entirely detached) as a result of the heater cartridge not being able to supply heat to the hotend anymore, or at least not as efficiently as if it was in its place.

Similar to the thermistor, even though the heater cartridge shouldn’t also really fall out of its place during standard operation, especially considering that it’s entirely possible to secure it tightly into its slot with a set screw, meaning that this issue is once again more likely to occur in cases where you have recently put your 3D printer together or disassembled the hotend, the fact that you will need to check the thermistor anyway practically means that you can check the heater cartridge at the same time to make things easier.

To find the heating tube of your Ender 3 / Ender 5, all you will need to do is to look right next to where the thermistor is on the heat block (you can refer to the video below to see exactly where it is, which is an official tutorial on heating tube replacement from Creality), where you should see two cables connecting to a tube.


Under normal circumstances, the heater cartridge should be fully seated within its slot without any part of it sticking outside and shouldn’t move or wiggle if you try to pull it with your hand, just as you can see in the video, and provided this is also the case for you, it would practically mean that the detaching of the heater cartridge isn’t what’s causing the thermal runaway error on your 3D printer.

On the other hand, if the heater cartridge seems to be coming loose or you’re unsure whether it’s tight enough, our recommendation would be to loosen the set screw holding the heater cartridge until you can move it, push the heater cartridge all the way in, and tighten the set screw while holding the heater cartridge in until it becomes fully locked in place, which will allow you to rule this possibility out entirely.

Thermistor / Heater Cartridge Damaged Wires & Loose Connectors

Provided that everything seems to be right regarding the heater cartridge and thermistor installation, with both components in their correct places, the next thing that can lead to the thermal runaway error you’re experiencing is an issue regarding the connection of the heater cartridge or the thermistor, whether it’s a damaged wire or a connector coming loose.

In the case of the connection between the thermistor and the mainboard becoming unreliable, the most common problem you will come across is fluctuating thermistor readings, as the thermistor won’t be able to correctly supply the temperature information to the mainboard and create a scenario where the firmware thinks that the temperature is going up and down all the time.

Similarly, if the connection between the heater cartridge and the mainboard is compromised, you will once again notice that the current hotend temperature keeps fluctuating, but this time, due to the heating cartridge actually not being able to keep the temperature stable throughout the print.

Regardless, considering that what the firmware sees in either scenario will exactly be the same, which is a fluctuation in the current temperature readings of your Ender 3 / Ender 5, whether it’s an actual fluctuation due to the heater cartridge working unreliably or one due to thermistor readings becoming erratic, the thermal runaway protection will eventually kick in and halt the printing process to ensure safety.

In this case, our first recommendation would be to thoroughly check the thermistor and the heater cartridge cables for any signs of damage, such as exposed wires, severe bends that can damage the wires inside, bending & breakage at the spots where they attach to the connectors, or black marks that indicate a burn, and to replace the affected component with a new one to be on the safe side if you spot any of these signs.

On the other hand, if you confirm that there isn’t any visible damage on the wires themselves, the next step you should take is to ensure that both the thermistor and the heater cartridge cables are connected correctly to their pins on the mainboard, in which case the connectors should fully cover the corresponding pins and become seated on the mainboard without any wobble, which can require a little pushing down to get right.

Misconfigured Thermal Runaway Protection

While rare, one factor that can lead to the thermal runaway protection kicking in even when there are no issues regarding the heater cartridge or the thermistor at all is a misconfiguration of the limits the firmware uses to determine whether it should activate the feature or not, known as the hysteresis and the period values.

The hysteresis value, in this case, refers to the maximum allowed temperature difference between the current temperature and the target temperature as the printing process is taking place, effectively meaning that any case where the temperature difference is lower than the hysteresis won’t activate the thermal runaway protection feature.

marlin thermal protection hysteresis


On the other hand, the period value determines the amount of time (in seconds) that needs to pass with the temperature difference higher than the hysteresis value before the thermal runaway protection feature activates and halts the printing process, preventing the protection from kicking in as a result of temperature drops that don’t actually last.

marlin thermal protection period


So, in a case where either of these limits is configured too tightly (usually as a mistake) to the point where they make the firmware think a thermal runaway is occurring during standard operation, where minor fluctuations in temperature readings can take place now and then, a thermal runaway protection error can pop up on your Ender 3 / Ender 5 even when your print is running smoothly, as also stated in the Marlin configuration file.

As an example, if the thermal protection period you’re using ends up being very short, such as a second or two, together with a hysteresis value that’s low, such as 1-2 degrees, a minor inconsistency in the temperature readings for a short amount of time can quickly lead to your Ender 3 / Ender 5 throwing a thermal runaway error when there is no need for it.

That said, even though increasing the hysteresis and period values will become necessary in cases where they are clearly misconfigured and keep creating false positives due to this, pushing them higher when they’re already correctly configured to refrain from coming across a thermal runaway error will effectively lead to the thermal protection feature not working as intended, or at all, creating a considerable safety risk where a thermal runaway can actually take place without the protection kicking in.

With this in mind, our recommendation would be to refrain from increasing the hysteresis to a value larger than 10 degrees Celsius and the period to a value larger than 60 seconds when you’re making adjustments, as a drop of more than 10 degrees randomly occurring during the print for longer than 60 seconds clearly tells us that something is wrong regarding either the heating or the temperature readings.

Thermistor Failure During the Print

If everything seems to be in order regarding wiring, configuration, and thermistor installation, a possible factor you should start taking into consideration is the thermistor failing during the print, especially if you have been using your Ender 3 / Ender 5 for a while now.

While the exact behavior of a failing thermistor, whether it’s fluctuations in the temperature readings or no temperature information getting supplied to the firmware at all anymore, will depend on the exact nature of the problem, which makes it impossible to directly point at a thermistor fault based on the signs, the instability in the current temperature data will, regardless, lead to the firmware activating the thermal runaway protection for safety reasons, similarly to any of the other issues we have discussed so far.

Since an unreliable thermistor is a considerable safety risk due to how it can cause your 3D printer to bump the hotend temperature up to unsafe levels as a result of the firmware thinking the temperature is lower than it really is, our recommendation in the case of a suspected thermistor fault would be to immediately replace the thermistor before powering your Ender 3 / Ender 5 on again to rule this possibility out and also ensure safety, especially considering that it’s not a costly component to obtain by any means.

Heater Cartridge Failure During the Print

Last but not least, if none of the factors we have discussed so far were the culprit behind the thermal runaway error in your case, a problem regarding the heater cartridge is practically the only thing that’s left, which, once again, is something to take into consideration, especially if you’ve been using your Ender 3 / Ender 5 for a very long time.

Considering that a failing heater cartridge can either create temperature fluctuations or stop heating the hotend altogether, depending on exactly what’s preventing it from operating as intended, a heater cartridge problem will look exactly the same as a thermistor problem to the firmware (and to us, which makes it hard to find out where the problem exactly is) due to the way the temperature readings behave in either scenario, leading to the thermal runaway protection activating to halt the printing process.

Since an unreliable heating cartridge is just as much of a safety risk as an unreliable thermistor, as it can easily lead to the temperature levels getting out of control unwarrantedly without the firmware having complete control over it, our recommendation in the case of a suspected heater cartridge would be to immediately replace it before printing with your Ender 3 / Ender 5 again, just as in the case of the thermistor, as this will both allow you to rule a failing heater cartridge out and also ensure that you don’t take any unnecessary safety risks.

Conclusion

While it’s not a fun experience to have your 3D printer halted with the thermal runaway error in the middle of a print, especially if the error was a false positive, the thermal runaway protection feature is there for your own safety at the end of the day, which makes it critical to ensure that it works as intended regardless.

With this in mind, our recommendation would be to avoid using your 3D printer until you locate the root cause and fix the problem, even though it can be a challenging process if you have never tinkered with the hardware of your Ender 3 / Ender 5 before, as it’s one that’s necessary for a healthy 3D printing process.