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Prusa MK2S main board FAIL and fire issue.  

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Prusa MK2S main board FAIL and fire issue.

I can't seem to add the pictures of the connector here...

So my main board failed the other day, I contacted Prusa CS but I guess I should have posted the issue here: Below is a series of emails (edited out personal info) from Prusa Customer Support

Here is my point:

The Heatbed main board Molex connector failed and the issue crept into the main board and it too has failed. I found posts going back to 2015 about this issue, it is a latent issue that should have been disclosed and attempted to remedy with every Prusa owner who has this board.

In my opinion here is what is going on:
The Mk2 inherently makes vibration caused by the liner metal bearings on metal rods, anyone who owns a Prusa can both feel and hear this harmonic vibration. I believe this vibration in time fatigues the tiny Molex clip (anyone know if these were Automotive approved?) that connects over the terminal post, not the plastic part, the metal unseen inside clip, this spring metal clip eventually separates a tiny bit from the post and thus arcing begins to occur, the post heats up of course and can fail as in my case at the mother board junction as well shorting and destroying the motherboard. The connector itself is supposed to be fire resistant; however the motherboard and certainly the surrounding area and materials are prone to burning. This is a serious issue.

What I concluded is NOT the issue
-it is not because the screws were not tight enough (that section of the connector is fine)
-it is not because the connector has come loose (sorry I am simply too up close and personal with my printer to not have noticed this).

What could be done
-certainly anyone with this issue should have their main board replaced if it has failed, frankly Prusa this is a latent issue you are of aware of and should be proactive on.
-any owner should be instructed, that after a certain amount of hours, should replace the Molex connector plugs on a regular maintenance schedule, this would prevent the tiny clip from metal fatigue and failing.

To Josef Prusa: please note the attached video and tell me if you approve of your customer rep as the last thing telling me to solder the connectors to the board and keep using it… are you serious?

The emails to and from Prusa –latest at the top.
Hello Shawn

Thank you for your feedback, but the system will not allow us to send spares for out of warranty printers and it does not apply to yours.
We already covered the maximum number of spare parts covered by the initial warranty. This machine's warranty period is over.
The board can be replaced if you wish by buying it from the shop or can be repaired by directly soldering the bed wires to the terminals.

Best regards


On September 30, 2020, 12:48 PM Shawn wrote:
Hi again
I did review that thread, the issue is the internal main board connection failed, this is not a matter of a loose connector or screws on my part. This is a latent defect that you are aware of and have not addressed, and the Main board is no longer usable as a result and serious issues could ensure.

To quote my email below again
" I'll be totally honest with you, I get and respect that things out of warranty are the consumers responsibility, but failures such as this that contrary to what you say are clearly a fire risk are a different matter in my book all together. The main board of my printer failed and failed in a dramatic enough way that could be a serious issue."

Perhaps you would like to pass this email and video on to Josef himself.

Like I mentioned, I am not a novice in the 3D printing community, and I did want to give you an opportunity to hear about this issue and perhaps take the high road and exchange my main board, I turn now to the 3d online social community with some details and warnings about in my opinion how serious this is.

Frankly I had better expectations for my original Prusa, shortly after purchase the LCD and heatbead were replaced under warranty as having issues, as well as the steel rods and bearings upgraded to the "S" and then replaced 3 times due to me having received the soft metal rods that quickly developed issues, (to that point my printer needed to be completely rebuilt 5 times by me) the power supply then failed just out of warranty and was replaced (at my expense) with another original as well as several printed part failures and extruder remakes after that, and now to have the main board fail... you get my point. Honestly in the time I have had this printer it has only been functional half the time, it has been great "when it works" and to date I have recommended it over my other printers for people wanting to deal with a company that stands behind what they sell. I do want to inform you though going forward I don't think this will be my recommendation any longer. Like I said, to pay what we do for the "Prusa" brand is to expect better and it is a huge disappointment that you can't deliver on this or In my opinion address this serious issue in a good will way.

I wish you well,

Hello Shawn

In this thread, there is a reply from Josef himself about this issue. Basically in the case of the Rambo board, it has original Molex connectors so you need to solder the wires of the bed to the board. This was improved in the Einsy board used in the MK3S.
Unfortunately, your printer is outside the warranty period so it is not possible for me to request a replacement for you.

Best regards

On September 29, 2020, 7:04 AM CST Shawn wrote:
First of all thanks for your response,
this was my first email to you that you responded to, the text of my second email is below and this is my third email to you right now.
The printer is inspected and "tuned up" at least every few months (its not my only printer either,)so this has not been an ongoing issue that could have been discoverable. …

Please see the attached video: this is after replacing the fuse and removing and replacing the part of the terminal that would still work for the hotend wires, the hotend and heatbed wires are NOT connected to the board right now, you can clearly see the failure point was at the base of the motherboard and the terminal joint, (as the burnt terminal part would show as well. The fuse would not fail again. The video I just took is after turning the switch on for just a second for the third time (just to catch the video), each time resulting in arcing at this point in the motherboard.

I'll be totally honest with you, I get and respect that things out of warranty are the consumers responsibility, but failures such as this that contrary to what you say are clearly a fire risk are a different matter in my book all together. The main board of my printer failed and failed in a dramatic enough way that could be a serious issue.

So my question is, seeing that the main board is not usable on my 3 year old printer, and considering how much I have boasted of expecting more for the merits of buying a Prusa printer, I'd like first to give you the opportunity to please replace this board for me as an "extended" warranty issue. Would you kindly do this?

Thanks for your consideration.


I sent the below email yesterday but have done some testing now
The arcing was NOT at the heat bed wire port plug that is tightened by the screw, that area is fine, in other words it is clearly not a build or vibration issue. The failure was in the motherboard under the socket and right up to where the plug is inserted, that entire area melted, it is clearly a failure of the design, to test whether the Rambo board is still good I temporarily soldered the heat bed wires direct to the underside on the Bed + and - pads, there is a short and arcing seen right in the motherboard between these pads, so it seems like the failure is in both the connector into the mother board and the board itself . The fuse was replaced and is fine.
Please advise me how to proceed:

Hello Shawn

The issue seems to be caused by a false contact that heated up to the point of burning.

It could have been a loosely held wire, corrosion, or even just the movement of the heated bed wire stressing the contact points where the amps consumed by the bed caused small electric arcs and slowly eroded the metal, causing it to overheat when the same current was going through a smaller contact area.

This is not a common issue but it happens, unless the board is inspected at least once in a year and all connectors are reseated to detect these problems earlier, see i3 Printers Regular Maintenance.

There is no risk of fire as the fuse blew as a safety measure to prevent further damage.

Let me know if you need further assistance.

Best regards,

On September 25, 2020, 2:56 PM CST Shawn wrote:
My heat bed on my original Prusa Mk2S stopped working the other day, the heat bed fuse blew and upon investigation this is what I found:
(I unsoldered the plug from the board)
I don't think this is a "wear" part that should need maintenance. I contacted Voxel Factory here in Canada and they suggested contacting you as they do not have this part.

Is this a common issue with a Prusa 2-3 years old? If so it should be rectified immediately as this is a serious fire risk.

Can you let me know how I can fix this please,


Publié : 01/10/2020 1:46 pm
New Member
Topic starter answered:
RE: Prusa MK2S main board FAIL and fire issue.

Publié : 01/10/2020 2:26 pm
Active Member
RE: Prusa MK2S main board FAIL and fire issue.

I am literally going through this very issue.  Replaced the molex connector once... which got me going for about a week.  Then melt/burn on the molex connector again.  So tonight I replaced the wires and the molex connector.  While my bed heats now, the molex connector grows unacceptably warm within seconds of turning the heat bed temp up and I have elected not to print at this time.  

My assumption is I have cooked my rambo in some fashion.   I ordered another board direct from ultimaker (seemed to be the cheapest price outside of china).

At no time did my fuse pop.  After the first time this happened I purchased one of those fire suppression balls which now looms large over my prusa.

I may try a direct solder to see if the original board is save able, otherwise it becomes my cnc's new brains with the 15A fuse removed.

Publié : 24/10/2020 12:43 am
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