Another Thermal Runaway and Power Panic Post.  

New Member

Hi everyone,

I've my Original Prusa i3 MK3S since Jan 2020 and only had minor issues. Today out of no where while printing the printer started going through power panic loop where it keep rebooting and the temp dropping when rebooted. Then I started getting both power panic and thermal runaway. It will print print a few layer and then I get the thermal runaway error or or the power panic loop until I stop it manually. I haven't made any alterations to the printer. I made sure all the wires are plugged in correct into the PCB, made sure that they were intact, and calibrated the PIN.

The only patter I notice is that thermal runaway  will sometimes happen right after the first layer is finished and the fan kicks on but its mostly random.

If anyone has an idea on how to fix this i would appreciate the help

Posted : 25/02/2021 6:21 am
Active Member

I am facing the same issue since today.

After ~5h of printing a PLA model I received the power loss error which I had never seen before (having the printer since November 2020). The print recovery started immedeately and the print finished without issues (less than an hour was left to finish the print).

About an hour later I started the next print and since then I receive a mix of power loss errors and thermal runaway errors. Sometimes the print recovers from the power loss and sometimes it gets stuck in a loop of recovering and power loss. When the printer is acutally running for a few minutes without power loss I notice that the nozzle temp is quite unstable; going up and down between ~208°C and its target of 215°C.

My printer is stationary and I do not carry it around or move it in its shelf so I do not expect any wiring issues. I checked the wiring anyway and cannot see anything wrong.

I am printing from SD and have no Raspberry connected. No modifications to the printer were done.

This post was modified 3 months ago by
Posted : 25/02/2021 5:45 pm
Active Member

I would start by examining the voltages on your PSU while the printer is heating up, then printing and see if it's drooping or dropping out entirely while under load. Tolerance should be within 5% of rated 12v or 24v depending on your PSU. 


You will need to check the power both at the PSU terminals and at the terminals on the board going to the extruder and heatbed.

Posted : 26/02/2021 3:38 am
Active Member

I measured the restitance of the heating elements and thermistors.

The heatbed at 2.8 Ohms seems quite low. Here it says 3.5 to 4.0 would be normal. The rest seems ok.

When heating up the bed I have almost 9 A of current drawn from the DC power supply (which is rated for 10 A according to its label). With the hot end joining I am a bit above 10 A which seems yet managable for the power supply.

When I saw this I thought that I found the issue but with increasing temp the current drops considerably. So when the motors join in the current draw of the heating elements is much lower already and the overall current is way below 10 A.

Also I cannot see a spike of current when the power loss occurs but the current clamp might be too slow to caputure a rapid spike.

The voltage at the power supply output stays at a constant 23.8 V the whole time - even during the "power loss". I have not measures the voltage at the board input as it is quite tricky to get in there while the printer is moving.


Posted : 26/02/2021 6:20 pm
Trusted Member

Do you live in Texas? 

Posted : 26/02/2021 10:38 pm
Active Member


Further tests showed:

Printing with the heatbed disabled I still get the power loss error.

Plugging off the power panic cable at the board results in thermal runaway error instead. 

Posted : 27/02/2021 7:08 am
New Member

Hi everyone thank you all for the posts. For me it turn out that heating element on the hot end was going bad and increasing the resistance. This would lead to too much power being drawn causing the panic. The thermal runaway cause  by element and fan. The heating element would have a hard time keeping up the temp as the resistance increased with the temp and when the fan kicked on and stayed on for a while it cause the error. Thank you all for the thoughts and ideas. 

Posted : 03/03/2021 8:33 am
Active Member

In case anyone is still interested:

My issues were caused by a defect RAMBO.


Being within warranty time prusa replaced the board but it was a real struggle to get there.

The supports depends on its customers doing measurements and documenting everything on video and pictures. There is no option to just send a part in and have the support do the troubleshooting.

So you as a customer spend a lot of time on documenting the requested tests. Then you wait for several days for an answer in which they ask you to do more trouble shooting.

Adding to my frustration was the feeling that my mails were not even read properly. Several times I was asked for information which I already provided in earlier mails.

In the end I had done a bunch of tests which all pointed to a defekt of the RAMBO and the support still wanted me to test other parts of the printer.

A very frustating process for the customer. I must say that I am highly disappointed by the support. 

Posted : 06/05/2021 7:10 pm

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