Suffering from thermal runaway.  

New Member

I purchased an i3 MK3S kit a few weeks ago and got a few good days of PLA prints out of it before trying out some different filament. I tried some Amazon basics PETG filament and noticed the print started to fail on me, with pieces of the infill not sticking. No problem I thought. I cancelled the print and tried again later and didn't watch as closely when I noticed the printer was stopped. It had the same failure in the print but it said thermal runaway.

Now, researching the issue as best I could for all this time, I often find mentions of the cabling of the thermistor or the heater to blame. I have almost entirely ruled those out through many tests, barring outright replacing them with different ones, which I will do monday when they arrive. The issue is consistent and downright repeatable and I believe has nothing to do with loose connector or broken wire. I think it has everything to do with the einsy board I have.

I have watched the printer consistently fail in the same manner. It will be printing fine for maybe 5 minutes, with the heatbed light on as well as the lights on the einsy board denoting the hot end heater being engaged all that time. Then suddenly both the heatbed and the hot end will cut out and temperatures will plummet and thermal runaway occurs. Neither the heatbed, nor the hot end will heat up again after thermal runaway, even if the reset button is hit, it will just end in PREHEAT ERROR. But a power cycle will make both function again like nothing ever happened, even without the print head and wires moving whatsoever. Moreover, if I have a thermal runaway and turn it off, then quickly turn it back on before it's cooled off and immediately start a print, it will print a shorter amount of time before thermal runaway. It seems almost like the einsy board (or a single component on it) gets too warm and cuts out the heater circuits. I have used a meter on the hot end connector on the einsy and just watched it go from doing its PWM modulation at up to 24v to an instant 0v with falling temps.

So many failed prints. Only completed one single object since it all started and it is nothing more than the triangle extruder visualizer. I printed it at the fastest speeds I could get the printer to go and it barely beat thermal runaway. It finished the print as the temp was falling.

I guess being a kit it's ultimately up to me to fix it right?

Posted : 23/03/2019 3:03 am
Honorable Member

try a factory reset and then start over at preflight page in the manual

the best prep for your bed is to wash it with warm water and dawn dish soap (in the UK use Fairy liquid) by using a wad of clean kitchen paper towel dampen with warm water and put a dollop of Dawn on the pad and scrub (i do it twice at 90 deg to each other holding the plate by the edge as it is a phonograph record rinse under running water and dry with clean paper towel keep touching only by the edge... I do this every other day of printing with both kinds of plate.

then set your live z with this method it is far better than the zig zag method in the firmware

if you still need help contact prusa online chat system 24/7 support
log into your account on one of the prusa store pages and in the bottom right corner you will see the Chat system box click in it enter your Invoice number for you Prusa and start the chat system they do an excellent job and will take good care of you.

Posted : 24/03/2019 1:29 am
New Member

Thanks for telling me about the live chat. I did not actually know it was there as I run very limited scripting on all websites. It was disabled so I never knew it was there.

Posted : 24/03/2019 2:43 pm
New Member

I also just finished my kit a few weeks ago. After several days of successful prints, I started experiencing thermal runaway's as well. Since I run Octoprint, I was able to monitor the temperature pattern on the hotend that occurred during the prints. What I was seeing was that the temperature would suddenly drop a various times during the print, sometimes not very much and sometimes a lot. If the temperature cannot recover within a fixed time period (I think 45 seconds), the controller will issue a THERMAL RUNAWAY error and stop the print. I believe what is happening (just a guess because I was never at the print when it happened), is that the fan over the nozzle is briefly over cooling the hotend at times.

I found posts that have suggested buying and installing a silicone sock to stabilize the temperature at the hotend, so I ordered a pack of three from Amazon that arrived yesterday. I installed one and retried a print that had failed four times. It finished without any issues and the temperature profile was much more stable that any print I have done so far. I haven't done enough prints yet to confirm that this was the problem, but at least the temperature is more consistent during the print AND it keeps the nozzle and heater block clean.

Posted : 27/03/2019 12:20 pm
Active Member

I too had a problem with thermal runaway after upgrading to the MK3S. It would happen on parts with a larger "footprint", after the first layer. The temperature normally drops after the first layer and the speed hotend movement increases. The Thermal runaway error occurs when the temperature readout of the hotend drops by 15°C for more than 45 seconds. I suspect that the new fan configuration improved cooling a little too much.

I posted about this in the "Assembly and First Print" section and @vintagepc suggested using a silcone sock. I did so and haven't had any problems since.

Here is more infor on Thermal Runaway:

Posted : 27/03/2019 2:03 pm
New Member

I had same problems after upgrade to MK3S. The problem could also be solved by increasing the voltage of the power supply from 24 to 26V. Does anybody knows if this results in other problems? (First prints are fine)


Posted : 07/09/2019 5:41 pm
Noble Member

It can also happen with the fan cooling off the bed faster than the bed heater can keep up. This is worse if with higher bed temps. Boosting the voltage is one way of dealing with this. Another effective solution is to insulate the bottom of the heat bed. Use some furnace insulation tape. It's about 2mm thick. Don't cover the entire bottom of the heat bed. Just a 5 cm square around the heat bed thermistor is enough to avoid the thermal runaway problem.

For a time, I had the entire head bed bottom insulated, but reduced it to just a small portion around the thermistor. On the other hand, insulating the bed does make it more efficient, but why did I stop insulating the entire heat bed? Biggest issue is that I need the heat loss from the bed to warm my enclosure. Also insulating the entire heat bed made cooling down way slower. 

This post was modified 11 months ago by guy.k2
Posted : 07/09/2019 6:11 pm
New Member

My first thermal runaway was when I first started to use PETG..I would start a print and shortly after it started printing I would get thermal runaway, this happened a few times and I finally figured out that when the fan came on it would cool the hottend and cause thermal runaway...I set the fan speed at zero but as soon as it would come on the speed was 255 so fast that it cooled the hottend and thermal runaway would occur.Now when I start a print I wait for the fan to come on and quickly turn the speed down to zero and the print will continue without a problem, then I gradually increase the fan speed after the print has taken hold on the bed and I get no more thermal runaway..Watch the hottend temperature as you increase the fan speed to be sure it's not dropping the temperature to fast, for me optimal speed appears to be around 160-180 after this all my prints ran good..So in the menu pick TUNE and drop down to FAN SPEED to adjust using control knob..Hey it works for me...

Posted : 21/09/2019 2:51 pm
Noble Member

Depends on which thermal runaway is happening (heat bed vs hot end). If it is a hot end thermal runaway, adding a silicone sock will help considerably. If you are printing PETG, you should be using a silicone sock anyways for protection against PETG strings badly fouling your heat block.

This post was modified 11 months ago by guy.k2
Posted : 21/09/2019 7:34 pm
New Member


Sorry for the late post, don't know if you still have this issue.

Personally, for small PETG prints, I have had the same issue.  Moving the print away from the center of the heatbed stops this issue.

Hope this helps.

Posted : 03/07/2020 10:52 am

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