Error The main thread is blocked

I have been using an up 300 for about a week with some pretty good results, however yesterday things stopped working with the error “The main thread is blocked”

I saw a post here before that said to try to restart after receiving the above error
after restart received the error “Internal USB connection error”
and after another restart and initialization “Initialization printer failed”

Another subsequent restart and i can get to the point where I can extrude some filament then the machine dies and the screen comes back with “The main thread is blocked”

Any help? or does the machine need to go back for service?

PLease contact your reseller or, you may need to change the PCB with the USB port on the back of the machine. This could be done by you or your reseller. Sorry for the problem.

Thank you for your reply, I have talked to the re-seller, he got me to unplug and re-plug cables on that board then confirmed that it will have go back and have the PCB replaced.

Thought I should update you guys here.
The problem ended up being the CPU, it was overheating and when it overheated it stopped communicating with the display board - and hence the errors.
Tiertime were very patient in working through this to get to the problem and I appreciate the support.
The Up300 is back to printing awesome results in ABS.

Hi @Sneaky, can you elaborate - was it the touch panel cpu that was overheating? It gets pretty hot inside the printer while printing, and all that heat pools in the top of the aluminium side where the touch panel sits. That would explain a few things if the case.

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Hello Sneaky,

May I know more about how do you know it is the CPU?
As we are also looking into the issue of main thread blockage and hope find a good solution for it.
If it is the CPU then this gives us a new path to explore.

Hi Jason,

I believe previously it was the CPU that overheated and subsequently died.

After the replacement the machine went well until 1st quarter this year.

After COVID lockdown here in NZ and with a firmware up date back then, the printer started to miss-behave - incomplete prints miss shaped prints, and nozzle blocking (this is using genuine Tiertime filament) I even ended up purchasing a new print head. I was busy renovating the house so didn’t have time to look into it.

Roll on to this week
With holidays and new firmware I thought it would be time to revisit.


Started off with some small prints with success - yay… well not so Yay.

Unfortunately I am now seeing that after about 80% print the error -Internal USB error - the main thread is blocked - this is on bigger prints like half a roll of ABS which is really annoying - yes genuine Tiertime filament.

One of the suggestions in the Forum was to reset the SD card - I cannot see this in a menu…

I have done a reset on the front panel

Now cycling through small to larger prints, and will let the forum know how I get on

Any help would be gratefully appreciated.


Ok yesterday and overnight did an 18 hour print successfully, will update the post on the next big print

Hmm, about 25% (140 grams of material in the bin) of the way into a 30 hour print and my friend -Internal USB error - the main thread is blocked - returned
I would say then the problem is not solved.

Ditto, on the not fixed. I have also replaced the LCD unit recently. I ran a 7 hour print on fine with no hiccups last night(thank goodness as it was the box for the gift this morning). Tried it on normal and it locked up twice. Main thread is blocked error. This is a pretty basic print-in-place spring box. I am running smaller version of the model on fine at the moment and its humming along nicely.

Also ugh on the waste of materials at this point…my son was gifted a ender 3 pro this morning. I expect once I start ‘helping’ him this may fall to the wayside.

And again, a single item print - ~130g in the bin, it seems to be after a certain volume of printing

Please provide:

  1. the touchscreen firmware version, indicate the alphabet of the final string (W or H):
  2. If possible also share the stl file for analysis purpose.

it is 1.5…14W

I just did a print that was 210g and came out OK!

Makes me want to be sure I am on the latest version… last was Using: as per my post
Sorry I cannot share the file as part of agreement not to share the file.

Another successful large print so failure does not happen based on how much volume it prints

Hmmmm 14 hours into a 23 hour print… and the main thread is blocked. I wish I knew what the delineator was so I could avoid whatever it is that’s f$@#ing up my jobs. This is highly disappointing.

Looks like the latest version available on the website is 1.5.14…I see you are on 1.5.15H, how do I get my hands on the .15H or W that you speak of?

Jason I do have a file you can have, where do I send it, If I upload it here will it be available to anyone on the forum?

It is interesting in that in one instance it failed at about 95% and the next it completed.


You can send to, we will analyze the file.

The H and W, actually means different types of electronics.
W type has lower hardware specs and only used in earlier batches.
It has smaller memory and slower CPU, therefore we suspect this type of electronics are more prone to main thread blocking issues and the we upgraded the electronics later to H type in later productions.
So if you have consistent “main thread blocking issue”, even with the latest touchscreen firmware, and you found that it is the W type, you can try contact support to replaced the touchscreen, but should make sure what you get is the H type!

Yes and it happened again 100g into a 620 gm job, Yes I need to replace the screen.