PID autotune fails 'Temp too high' with 12 V heater cartridge but works with 24 V?
It is all about resistance. 

This requires some formulae:

$U = Voltage$
$I = Current$
$R = Resistance$
$P = Power$

$U = I \times R$

$P = U \times R$

$ R = \dfrac{P}{I^2} = \dfrac{U^2}{P} $

The 12&nbsp;V, 40&nbsp;W cartridge has a resistance of about 3.6&nbsp;&ohm;.

If you use this cartridge at 24&nbsp;V, this caculates to a power of 160&nbsp;W!

This means that there is an enormous influx of heat that is hard to control, hence the overshoot.