<p>The heater cartridge on my CR10 V2 broke so I ordered a new one(12V). After replacing it the new one heats of very quickly past the target temperature and the slowly decreases to the target. Once it hits the target temp it goes up and down by 10&nbsp;&deg;C or so during prints. During the prints there is under extrusion. I read that if the filament gets hot too high up the hotted it could cause clogs. I replaced the nozzle as well and cleaned out everything.</p>

<p>Some things that I think may have an effect:</p>

<ol>
<li>Could the new heater cartridge not be compatible?</li>
<li>does a poor solder job with heater cartridge wires have an effect</li>
<li>It's possible I messed up the thermistor when replacing heater cartridge, could that explain whats happening?</li>
<li>Is there anything else in the hot end assembly such as fans that would cause this?</li>
</ol>

This sounds as if you have bought an incorrect heater element, e.g. one for 12 V instead of 24 V. The CR-10 uses 24 V. The 12 V cartridge has a lower resistance, so when powered by 24 V, the current is much higher and therefore also the heating power (
$ {(\frac{24}{12})}^2 = 4 $
times higher). For details on the calculation, the
this answer
on question:
"PID autotune fails 'Temp too high' with 12 V heater cartridge but works with 24 V?"
. This makes the hotend heat up very fast resulting in a large overshoot. You need to replace the cartridge for one for 24 V.
Note that I recently experienced exactly the same problem by mixing up the cartridges see
this answer
.