I was replacing my printer's hotend, and tested it out by printing a calibration cube. But the commands wouldnt work, and I found out whenever I open Repetier-host the extruder temp. reading would be at 478 degrees Celsius, which is def not the case since my hotend is cool at touch. I also reloaded the marlin firmware, but nothing worked, could there be a line of code to change this in the firmware?

Some other posts suggested that the thermistor was dead, but after replacing it, the same readings would show. I've been working at it for 3 hours now, but just cant solve this issue!

I had similar issues. My thermistor legs were being shorted at the hotend due to an overenthusiastic technician at the place where they put kits together.

Check the resistance of the thermistor leads when not connected to your RAMPS board. If they are around 1-2 ohms then you've got a short. If they are higher than 50 ohms then the thermistor is OK.