Temperature readings from hotend thermistor wrong/inverted

So I have the ratrig v-core 4 400mm. Did most of the hardware aaaaages ago and only got around to trying to configure the software recently. At powerup the hotend cooling fan ramps up as high as it'll go, and reportted temp in the web interface is 130C. I've removed the jumper to see how that affects it and it then hovers at 150C. Heating up the hotend caused the temperature to drop rather than increase. Is there something configured wrong somewhere?

I've also measured the resistance when not in the system and am getting around 1kOhm, same as another rapido hotend that i just took out of its' box