Hotend Not Heating - Several Troubleshooting Steps Already Attempted
Hey All,
I've had a Prusa MK2S for a bit over a year now and it has proven to be a reliable workhorse. However, I recently have been having issues with the hotend. Specifically, it started as failing to maintain heat during a print, occasionally leading to a "THERMAL RUNAWAY" error. It wouldn't always though I could see the temp drop off on the Octoprint graph before going back up.
When the "THERMAL RUNAWAY" issue happened more than once, I checked all of the connections to the board and looked for break in the wires and found nothing amiss. I replugged all of the connections and went from there. Things worked for a bit before the same problem reared its head.
I concluded it may be the heating element failing as the Ohms measured a bit high. I ordered and replaced the heating element and all was good for a few days with it finishing prints as expected. Today, I have once again come back to find a print failed halfway through with the THERMAL RUNAWAY error up.
At this point, my guess would be an issue with the Einsy board since I'm not sure what else to check. Everything I have seen with the thermistor looks okay. This was tested by preheating the printer to PLA. The thermistor temperature read properly and even went up slightly as the bed rose to temperature. However, the hotend did nothing. Sometimes it works on the second preheat attempt but there is definitely something wrong.
What do you think my next step should be? I've done quite a bit of searching on Google and this forum but haven't found a good next thing to try. The fuses look okay and the random nature of the event makes it difficult to troubleshoot. Do I just need to bite the bullet and order a new board? I have pretty much whatever tool I'd need to complete tests you can suggest.
Thank you very much for the help!
-Alex
Re: Hotend Not Heating - Several Troubleshooting Steps Already Attempted
The only way the printer knows there is a thermal runaway event is if the temperature passes some level; and it only knows this using the thermistor.
So it's a failed thermistor for my vote. They can look perfectly good, measure perfectly fine, and still be bad. It takes a full time at temperature diagnostic to really know if they are good. And it seems in your case your thermistor is failing the time at temp test. And since these failures are pretty random, it'll be hard to catch it in the act.
Re: Hotend Not Heating - Several Troubleshooting Steps Already Attempted
Thanks for getting back to me Tim. I ordered a new thermistor which should arrive in the next few days and I hope that will resolve my issue.
I'm not yet 100% sold on this being the problem as a few examples don't lead me to the same conclusion. For example, when starting from cold, both the bed and hotend thermistors read the same or within 1 degree of each other. I will start the preheat and watch the bed temp climb while the hotend stays the same or oscillates slightly. I will then restart the printer, preheat again, and now both temps will begin to increase as is normal.
My understanding of the THERMAL RUNAWAY is the printer is entering a protective mode when the thermistor reading is either too high or too low. The issue isn't that the thermistor suddenly reads 0 degrees or 300 but rather that it simply slowly drops in the exact way it normally does as the hotend cools down: quickly at first and then leveling off as the temperature difference between the hotend and ambient area shrinks. Sometimes it drops low enough to activate a THERMAL RUNAWAY error and sometimes it suddenly climbs (not shoots) back to the temperature setpoint.
I'll be swapping the thermistor regardless as to not leave an stone unturned but I'm interested if you or anyone else has any other ideas.
Thank you for your help,
-Alex
Re: Hotend Not Heating - Several Troubleshooting Steps Already Attempted
As I understand the THERMAL RUNAWAY error, the software is monitoring the thermistors, and if the temperature reading changes too quickly, it triggers the failure. A damaged sensor can act something like a switch when it fails: one second fine, next second nothing or a strange reading, and a few seconds later fine again. Rate of change is the thing they are looking for. And, thermistors in hot ends, from my reading, tend to be fragile since they are already being used near their spec limit temps.
That said, what you describe, that you see temps start to drop in a more or less controlled fashion, sounds more like a heater or heater controller issue. But the failure mode in that case seems a bit weird. Mosfets and drivers usually just die open or shorted - intermittent isn't something they do well... lol.
Assuming you replaced the full heater: cable, connector and all; have a look for a poor solder joint on the pins of the board side connector. If any of the pin joints look dull or muddy rather than shiny, or a pin "wiggles" a bit or has a 'ring' in the solder like the following pics ... bad joints caused by long term thermal stress generally stand out from neighbors. In extreme cases the glass epoxy near the pin will discolor. If the pins look good, then I'd look at power limiting (fuses operating near their rating can become intermittent - or the PS could be current limiting), and then perhaps bad driver electronics.
ps: all these pins look suspect to me and deserve attention; not just the more obvious ones marked in red.
Re: Hotend Not Heating - Several Troubleshooting Steps Already Attempted
Tim,
That's a really good suggestion and something I hadn't previously considered. I removed the board from the printer and thoroughly inspected all of the solder joints. Unfortunately (or fortunately) nothing looks suspect to me. I've attached an image of the backside of the board as well.
While waiting for the new thermoster, I've also completed a second, more thorough inspection of the thermistor cable from the cartridge to the connector and all looks well. I'm hoping that is still the problem and will have the replacement in on Monday. In the meantime, I will reassemble and test the printer. I'm hoping I can reproduce the symptom of starting a preheat and the extruder not heating to get a voltage measurement across the extruder leads to see if there is every a time that the board isn't properly outputting 12V.
Thank you again for your help in attempting to solve this problem!
Re: Hotend Not Heating - Several Troubleshooting Steps Already Attempted
Attached is the backside of the board.
Re: Hotend Not Heating - Several Troubleshooting Steps Already Attempted
Photo may show something: look with a loop or magnifier - make sure that dark spot on EXT- isn't a broken trace. Or ohm them.
Even EXT+ and BED+ look off - but it could be image artifacts. I believe the small pads are test points.... but who knows if other routes connect to those vias.
Also, check this point - the wire bridging the two pins looks out of place.
