Extruder heating starts just after initialisation and never stop!

The extruder of my UPBOX printer starts to heat up as soon as the printer is initialized. Without any heating command being sent. And it starts to heat without stopping at more than 350°C which burns the ABS.
I changed the electronic board at the back of the print head, the heating horn and the large white electrical ribbon. The problem is still there.
What can I do? The machine only printed 6.5 Kgs
Nicolas