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I used to be able to print with my Max V3 for 30+ hours. Now after about 2 hours I get the def on the temperature display and the print is gone. I have also noticed that the nozzle and bed temps appear to be varying much more than normal. This has started to degenerate within the last 3 or 4 days.
Any ideas for what is causing this? What can I do to fix it? Thanks much for any insight.

If the temperatures are jumping abruptly up and down you have a thermistor wiring problem. If it's going smoothly, check wiring, drive voltage, and PID.

Thanks. How do I check the drive voltage and PID? I bought the Max V3 pre-assembled. Where is the best source of information on how to do this?

OK, figured out how to autotune pid. Did it for both hot end and bed. Things look really good right now.

I spoke too soon. It went def again after about 7 hours. Not sure what wiring to check out. Any hints would be appreciated.

So, the wiring most likely to have been damaged is the hotend wiring. I would do a resistance reading on each wire for the thermistor and heater cartridge, and see if the resistance changes much as you move the hotend around the build area. if you find one that does that, replace it. Once you've gone over all of them, try it again.

Another source of issues is that in the northern parts of the world, temperatures are dropping, so if there are drafts from outside the house, or just drafts in generally, they may be getting cold enough to actually overwhelm the temperature management, so a quick fix would be a blanket, dropcloth, or cardboard box. Longer term, you may want an enclosure. There's plenty of topics on here about them, so check those out for more info.

So it turns out it is in the thermistor on the bed (nozzle may be an issue too, I will keep a lookout for that as well). I went over all the wiring, reseated everything I could. When I got to the bed I noticed that the kapton tape over the thermistor in the center of the bed was loose. I put another strip of tape over it and that seemed to stabilize everything. Got a def again after an 11 hour print and was about to panic. Went to the bed pulled off the old tape and put a couple of new ones down and noticed that the thermistor was way up above the bed. I pushed it down and things looked good for a second. When I turned on the printer the bed showed 366 degrees and then everything went def. After a few times of going back and repositioning the thermistor things appear normal again. Doing some more prints and crossing my fingers.

That's useful info. High temperature is low resistance. You're probably looking for someplace that's not insulated properly and shorting out. Could be the thermistor itself or anywhere the two wires to the thermistor could touch each other.

This has become really weird. I am getting the def after about 30 minutes. What is weird is that after I press the reset button to stop the print so the heaters turn off, it then reports the correct temperature again. Of course, the print is ruined at that point, luckily it didn't get very far. I am assuming something is wrong with the thermistor and will order a new one. Other insight will be greatly appreciated.

The tricky thing about intermittent electrical issues is that they're intermittent. You get something making contact that's not supposed to make contact (or something not making contact that is supposed to make contact), but only when just the wrong vibration happens in the machine. Consistent issues are easier to troubleshoot. Obvious candidates here would be whether the PTFE insulation on the thermistor legs goes up far enough to prevent the thermistor legs shorting each other out next to the bead and whether the joints between the thermistor legs and the wires are properly and robustly insulated.

Make sure you've installed the thermistor as shown here: https://www.youtube.com/watch?v=i6yl74TzBxY - it's very important that the thermistor legs are covered with PTFE tubing.

g.

Thanks. I was careful with the PTFE tubing. I noticed several things that may or may not be relevant. First the temperature stops to dip and won't go back up just before the def appears. Secondly I noticed that the solder for one of the white wires that connects to the mainboard didn't look too shiny. I fixed that. If that doesn't work I will review the ptfe tubing on the thermistor. If that doesn't work, I just ordered 2 new thermistors (just in case). One of these days I will get to the bottom of this.

So I finally figured it out. One of the pads for the thermistor separated from the bed. The Kapton tape held it down, but not that well. I have tried to solder it back, but the solder won't hold since the pad is gone. Anyway, I ordered a new bed. That should do the trick.

Did that work for you? I had this problem when I upgraded my v2 with an accelerometer and whip. Turns out that while printing the connection between the accelerometer and the whip would wiggle loose during sharp movements. Not enough to free the whip from the accelerometer, but enough to temporarily disconnect the hotend thermistor giving a def error. I printed this retention clip https://www.thingiverse.com/thing:2192079, and used a zip tie, haven't had a problem since. Once I even had a 30-hour print, no problem.

I think I remember the hotend would occasionally lose heat as well, does that sound like the heat fluctuation you mentioned?

So, replacing the heated bed worked until now. Last night, tried to start a print. Was going well for about 30 minutes then def appeared. I shut down everything and went to sleep. So now I am trying again. Now the bed just won't heat up. It shows the bed temperature but never changes. I suppose that means I need another hotbed. This is becoming tedious. I wonder if anyone has any clue about why my hot bed would suddenly die. I took it out and inspected everything which looks fine...

Check the heated bed fuse with a multimeter. If that checks bad, replace it. If it checks good, begin standard diagnostics... Voltage at the board, voltage at the bed...

Also check the connector on the rambo to make sure it's well tightened down. If it's come loose at all that will cause it to eventually melt down.

thank you for all he tips here, i've been having the same problems so i was searching for solutions. i'm also having health problems searching for drug reviews but i guess that's another topic. anyway, thank you once again a lot!