How do I diagnose an unresponsive Lulzbot Mini


My Lulzbot mini has been a great printer over the last few years, I have probably put 40 Kg of PLA/ABS through it. Yesterday I had a broken thermistor wire, so I unplugged the machine, removed the print head, and replaced the thermistor as per instructions online at After installing the thermistor I plugged it back in to my computer and turned it on. It connected via USB no problem when using the latest version of Cura - 2.6.69. However it is unresponsive to manually moving x, y, z, or temp changes to bed or nozzle from the Cura interface, and unresponsive to “start print”. I also sent it simple move G-code commands from the printer control in Cura and it didn’t do anything. If I unplug the USB then Cura recognizes that the printer is not online. When I plug it back in and hit connect then it says it is connected via USB. But apart from that little handshake I can’t get the printer to do anything.

So what are my options to determine the next step?

How can I check if the Rambo board or the Uno are the problem?

How do I diagnose the issue?

Many thanks in advance.

I love this printer and this is the first real issue I have had with it in 2 years.



I had a similar situation myself not long ago. I had my new Mini up and running for about 1/2 an hour, then it stopped cold mid-print.

The Rambo board still registered to the PC, but the printer was otherwise unresponsive. Lulzbot’s been kind enough to send me out another, but in the meanwhile I’ve borrowed one from another machine to get some work done. Installing it, I have the same issue: I see it on the PC (The Rambo board), but not connecting otherwise. :question: :question: :question: A dead-giveaway that this is the case is when it fails a firmware update.

So, after confirming I have 24VDC going in, and double checking the cables, I checked the 5V rail on the ICSP lines, which was reading only 1V. Like if the regulator was being shorted out. Weird. So I start unplugging all the end stops, one-by-one and checking the voltage (pwr off; unplug a cable; quick-turn-on; read-5Vline; quick turn off). (edit: I also checked continuity on all 3 fuses - A-OK)

The culprit seems to be the UNMARKED Green/White wire set that I can only assume that’s supposed to be connected the “Z-Min”. It powers up FINE unplugged, but as soon as I reattach this to the Rambo Mini, it shorts out. Remember - the Mini now uses the touch/tap to establish the Z-Min, not a mechanical endstop.

That’s where I’m at right now. No solution, but to me it sounds like you’re in a similar situation. The 32U2 (the USB bridge) is powered by your USB port, so you can see it, but the 5V rail that powers the ATMega sounds dead like mine. Perhaps start pulling harness conns out like I did until it comes back to life?

If I come up with a solution, I’ll post it.

Good luck,

Further update. I’ve traced it down to a short between my “5V Fan” header and ground.

Plugging in my Zmin puts a ground connection to the 4 corners on the table.
Plugging in my 5V Fan’s 5V line shorts to ground.

If I remove either of these lines, 5V is restored, so the issue lays somewhere between these two lines touching…someplace…

Gotcha! Took off the extruder to inspect the wiring, and found this. (Sorry, apparently the "Board Attachment Quota has been reached, and I can’t upload my pic).

The 5V fan power lead was resting on the edge of the hot-end, and eventually melted through to short out the 5V rail of the whole motherboard. Eventually, the power 5V regulator gave out, killing my Rambo mini.

A little kapton tape and some re-routing of the 5V fan leads, and I’m back in business!