This is one of two things
1) a shorted cable to the hotend thermistor, unlikely since you have replaced them, but unplug the thermistor cable from the control board, the temp should change to about 0
you can also plug the hot end thermistor into the bed thermistor plug and verify the bed reads room temperature, verifying that there is nothing wrong with the hot end thermistor
2) a dead controller port... (needs a new motherboard)