1998 buick century--always blows hot no matter how low the temperature is set--ac or vent makes no difference--as if inside cabin temperature sensor is broken/giving bad data

Is the compressor running and if so, have you checked the low & high side pressure?

haven't checked compressor---blows hot even in 'vent' mode which should bypass compressor and use outside air only

SOunds like the trap door isn't shuting to or opening to control from where the air flows ( heater core ,,, or AC evaporator )