OK, so now you're getting the MINTEMP error. This suggests the hot end thermistor isn't connected to the RAMPS board where MARLIN expects it. (MAXTEMP indicates the thermistor is shorted out, MINTEMP indicates the thermistor is open or not connected). Is the hot end thermistor plugged onto the T0 connector on RAMPS, and the bed thermistor (if you have a heated bed) plugged onto T1?