Trevo_B23

Hey guys, i have a 1990 subaru legacy wagon that i bought about 10 months ago. it was running great up until about a month ago when it overheated the first time, i drained the radiator and put new radiator fluid in it and it was fine. well two days ago the heater started blowing cold air on the hottest highest setting, and the engine overheats. I'm not sure what it is i checked the trouble codes and 21(coolant temperature sensor) is being thrown but i previously switched out that sensor three months ago. Any suggestions would be helpful. also i am new to mechanics so please dumb it down a bit. Thank you