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&nbsp;&deg;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&nbsp;&deg;C) and lower (150&nbsp;&deg;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)?
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.