Heating problem

[lulu]

Hey guys,

i recently got my first 3d-printer, a Tevo Tarantula with two extruders. It's all assembled and kind of working.
Every time i try to heat the hotend, it stops at a random point but always before my set temperature.
Here is what i tried to do:
Rewire the thermometers
i tried to print something hoping it would heat up (the printer did the neccesary movements but did not heat)
heat with the display and controler of the printer itself
heat with my laptop via Repetier Server

Every time it stops and can only start to heat up after i unplugged the printer. It also sometimes tells me "heater decoupled" out of nowhere.

Can somebody help me with this?