Hotend heating on powerup

Ive just switch the stock hotend on my ender 3 to an e3d voncano.
Just swapped the heater cartridge and thermistor, so no changes to the wiring at all.

When I turn on the 24v supply the hotend immediately starts heating!
It got to 300C before I noticed the smell the first time. Thankfully nothing seems to be damaged, but I cant figure out what's causing it.
Any suggestions?

I did have a thermistor issue when I first put the new hotend together. It initially read -14, but after removing and reinstalling it was fine. I assume I must have shorted the tiny little wire on the heater block or the screw that holds it together.

Running marlin, and as far as i know it got all the thermal protections enabled. Guess these cant save you from a short circuit or dodge mosfet tho.

Think ive just found at least part of the issue.

I noticed the temp wasnt dropping below 50c even after it was left off for ages. Checked the voltage on the heater, and it was 3.3v.
Is this a sign of a blown MOSFET?

Im using a custom-designed hotend mount with the wires tucked into a small hidden channel, maybe the thermistor has shorted across the heater.... I'll pull it apart tomorrow.