I decided to inspect the heatbed and removed the black plastic cover at the back (something I had never done before). To my surprise, I found that one of the thermistor cables was likely pinched from the factory by a screw, as seen in the pictures:

At first glance, nothing appeared broken. To be safe, I covered the exposed wire with electrical tape. Then, I measured the resistance at the thermistor connections and got 108 kΩ, which, according to the troubleshooting guide, is within the expected range.

I also checked the heatbed cables, and they measured 5.5 Ω—lower than the 6.5–7 Ω range specified in the guide. This makes me suspect that the heatbed itself may be faulty. Now I’m wondering: could the exposed thermistor cable have caused this issue?

It’s frustrating to find a factory defect like this, especially since I had never touched this part before. If you’re experiencing similar errors, it might be worth checking your thermistor and heatbed connections. Hopefully, this helps others avoid unnecessary troubleshooting!

Thanks for the help!

Posted by: @joantabb

Yes the thermistor use not polarity conscious  so replace the wires making joints inside the controller case and beyond the clamp at the other end

Thanks.  I just ordered the wire mentioned earlier in here and found 2 complete replacements in the US on ebay.  I'll see about splicing the existing one under the bed and in the box and use a cable support listed here  Someone who uploaded that make had spliced wires in the picture.  I guess while it's down I can install my bondtech IFS and 1-piece PTFE passthrough mod.

I keep a stock of flexible silicone wire in 24AWG, for replacing thermistor wires, Motor wires, pinda wires, fan wires and filament sensor wires. (Various colours Red ,Black, Yellow ,white  green and blue will probably be enough colours. don't forget heatshrink tubing, soldering iron, solder and zipties...
I also keep 16AWG for heatbed power and extruder heater wires. (RED and Black) 
I don't need to use them frequently, but they are available when I do need them

As I said, you probably won't need them frequently, but they are handy to have on hand. 

Nozzle thermistors and heaters are also useful to have 'in stock'!

Joan

Sorry to bump this but having the same issue.  Super pissed about the possibility of having to wait for a new thermistor to come in.  the wires don't even look damaged.  Would it be possible to just replace the wires and not order a new one?  If so, how are you disconnecting the old wires?  just cut and solder?  After I get mine up and running again I'll return here with an update.  

Thanks

The kicker is that my Mini + had been printing beautifully!  Replaced the nozzle with an E3D Nozzle X and have been printing CF PETG, it’s amazing stuff. 

1. Replaced the thermistor wires with high strand count, silicone jacketed wire.  This wire should do much better with long term flexing.
2. Eliminated hitting the back of the box and eliminated tight bends by replacing the cable strain relief with this design.

Ask me in another year on how well all of this is doing.  :>

The new parts:  https://a360.co/2Ycb4i5

@Mr.Shaky,

I'm functional in F360, but I can't figure out how to download the models for these parts.  Clicking on the link brings up a markup view in the browser.  How do I either download the model or open this in F360 itself?

Thx!

Its been a year since the last post in this thread, so there should be good run time on possible solutions:  A) Modified strain relief (angled wire guides) at the bed-end; B) e-chain, and C) silicone wire replacement.  (did I miss any solutions?)

Questions

1)  Is your solution still working?

2) What else should I expect to fail on my MINI?  Is the bed thermistor by far the most common failing component?

I have a bed thermistor on order, but I suspect that the problem is wire fatigue, and changing out the wire would be the best move. 

1) It is wire fatigue;  the chance of bed wire failure is greatly increased in enclosures where the bed wires impact a side (mine is a touch over a 17" cube.)  I haven't had any new wire issues since creating/installing the angle tube and modified board cover, having minimised the amount of bend/flex the wires are subjected to.

2)  Another potential source of grief is the heatbreak.  Because of the design compromise to make the printer more-affordable, the tube OD does not have a pinched-in section above the heat block, so heat creep will cause the ptfe tube inside to melt/shrink/constrict over time, causing filament feed issues.  ABS/ASA and other high-temp prints will accelerate the failure timeline.  Bondtech sells a replacement heatbreak, though I hadn't seen much news regarding long-term effectiveness.  

The alternative is to replace the hotend.  E3D and Slice Engineering offer replacements; I went with the SE Copperhead for my light-duty needs and very minimal impact on print volume/geometry.  (There's a thread regarding all this; I didn't post in it and it's been too long so the location dropped off my browsing history.)

Optional mods:  Bondtech extruder with filament sensor.  Bed leveling mod

Posted by: @cjnaz

Its been a year since the last post in this thread, so there should be good run time on possible solutions:  A) Modified strain relief (angled wire guides) at the bed-end; B) e-chain, and C) silicone wire replacement.  (did I miss any solutions?)

Questions

1)  Is your solution still working?

2) What else should I expect to fail on my MINI?  Is the bed thermistor by far the most common failing component?

I have a bed thermistor on order, but I suspect that the problem is wire fatigue, and changing out the wire would be the best move. 

 

Mine is still working at 2 years.  Between it and the part cooling fan, these are the two most common replacements for me.