# PID autotune fails under all conditions so far. Any ideas I haven't tried?

I got some Prusa i3 clones with Melzi boards with Marlin, or nearly so. One of them performed very well (considering that the board would reboot whenever power was applied to the build plate. But I digress) until finally the hot end failed, possibly due to me running it for two days with little respite. The replacement, a typical MK8 clone, installed almost painlessly but failed to start due to over-temperature conditions (we're talking as much as 45 °C over set temperature). I tried the known solutions, continually reducing my P value (got down to 7 before I gave up) and attempting autotune. I also tried autotuning to both higher (230 °C) and lower (150 °C) temperatures.

None of these seemed to produce an autotune successfully, and the same error message temp to[sic] high appeared in all cases. Can anyone suggest something besides a new hot end (I have one on order, but what if I have the same problem with another new one)?

• Maybe you can find some hints in this thread? – 0scar May 23 '19 at 14:51
• Nope. It pretty much tells me to do what I did (and wrote about above). I may have left out a few hours of details, though. I am going with the idea that my replacment is a 12V hot end, and the requirement is 24V hot end. I bagged one and will try it this evening...and tomorrow will update. Watch this space. – Peltier Cooler May 23 '19 at 19:55
• You can manually tune the PID. – Jeroen3 May 24 '19 at 6:45
• Yes I know. Turns out I'm using a 12V hot end and should be using 24V. I looked up the resistance to be sure; so no amount of tuning the PID would fix that – Peltier Cooler May 25 '19 at 3:02
• You can always just limit the maximum PWM rate to supply to your 12V heater, via #define BANG_MAX 64 64 would have a 12V cartridge run in a 24V system as it if was a 12V system. Make absolutely sure to enable thermal runaway protections, since a 12V cartridge on 24V can get very hot. – towe May 28 '19 at 11:30