MAXTEMP error

Hi, I built my first printer and when I start it up for the first time I get the following error...
Error:0
: Extruder switched off. MAXTEMP triggered !
Errorrinter stopped due to errors. Fix the error and use M999 to restart. (Temperature is reset. Set it after restarting)
ok T:0.0 /0.0 B:0.0 /0.0 T0:0.0 /0.0 @:0 B@:0

and when i check the temperature it says...
ok T:453.1 /0.0 B:0.0 /0.0 T0:453.1 /0.0 @:0 B@:0

So my problem is the heater if reading 453 degrees without even being on and regardless of if the thermoster is plugged in or not. I'm well educated in the computer field but not electrical and wiring so I'm not sure what to do. I have a voltmeter and can do any troubleshooting necessary if someone could help me out with what I can do it'd be very much appreciated.

If its reading 453 while unplugged its a big issue

With the thermistor is unplugged it should read 0C, means its an issue with the board or firmware

As these devices warmup their resistance decreases, so to be reading that high a temp the board is probably shorted somewhere. Check the ramps and the mega board carefully for solder blobs and joined pins

If you compiled your own firmware, check the motherboard is set to your actual hardware, or the pins will be wrong and you ill also get strange readings on the temperature.