Hotend setup

Hi, I am trying to connect a E3D V6 Extruder J-head Hotend to a MKS GEN V1.4. I have my problem is that when I use the command M105, I get a temperature reading of 355oC for room temperature. When changing #define TEMP_SENSOR_0 from 1 to several different values like 2,3... , I get even strange values in the range of 700 to 800oC. If I try to set the temperature using M104, I immediately get an error saying that the temperature exceeded the maximum. If I change #define HEATER_0_MAXTEMP from 275 to, say, 1275, I don't get the error message immediately. However, after a few seconds a different error message comes and shuts down the board.
I think I am missing something obvious here. Could anyone please point me in the right direction? Thanks,
Matheus

reading that high normally indicate that the thermistor wires are shorted together somewhere. Normally at the hot end, often by the thermistor retaining screw.

Well the V6 uses a semitec (Marlin option 5) thermistor with a max temp setting of 285C, but since they've switched to cartridge type, are you sure you ordered the thermistor version and not the PT-100 version? (they're very similar looking).