12 volt heater on 24 volts. YIKES! SNAP, CRACKLE, POP!
I had a lot of trouble with heaters and thermocouples when I first started. I did have some trouble with the wire for the T-Couple breaking. I ended up putting the T-Couple wire along side of the heater wire, inside of heat-shrink tube, making sure I didn't get too close to the heater. Maybe, 2" or 50mm.
Another little thing I did, was to put some Heat-Sink Grease around the heater, T-Couple and nozzle.
That made a big difference for Me. Just a small smear is of Heat-Sink Grease goes a long ways.
It has helped me a lot when I have a problem and I see someone else having the situation. Then there's the answer, within the next few posts.