No heat (computer climate comtrol)

Hi
In first, i m fench so i must say i can't write in a very good language...sorry !
It's k to read posts but writing it's harder
I got a problem with my Blazer (4.3 LT 1999 105m KM)
It s impossible to get heat air
The A/C works ok but its only cold air whatever temp i select
I precise my blazer gots the automatic control
i have read another threads about HVAC but none seems have the same problem as me
After my reading, i wonder if its my blend door which doesn't work or a temperaure captor/sensor
What do you thonk about y problem and can you help me ??
i hope !
thanks for you answer

ckeck your coolant level in the radiator. also you might need to flush out your system. if your heater core gets clogged, you wont get hot air. well this is the most common problem so check it first.

May be worth putting a thermostat in. Mine wouldnt give me hot air in the winter, My t-stat stuck open there only a few dollars and could fix your problem