not enough info..problem could be electrical..relay,switch or PCM ,not signaling compressor 'on' or the blend door may not be changing position (controls airflow through heater core & a/c evaporator) symptoms ? noises ?any other issues with engine?? any clicking from comp. clutch when a/c switched on/off? .Also if heat is blowing through te vents with the fan on high , turn the temp dial from hot to cold and back again . Is there any noticable difference in the air temp? If so , then you have coolant going through your heater core (normal) If it was air going through the heater core then you would most likely be getting air through vent closer to ambient (room) temp .