The air will not switch between cold and hot whe you turn the control nob on my 1999 Chevrolet Silverado 1500 Z71.

Just for fun,clamp off the heater hoses,just squeeze them flat in place to stop the hot coolant flow through the heater core.Take it for a spin and see if the a/c stays cold.Sometimes with increased hot coolant flow through the heater core with increased engine rpm (driving) and a leaky air flow door in the a/c heater assembly,it can be hard to keep the a/c air cold.

If the orifice filter is clogged, this will happen, a better check can

be done with an ac manifold gauge set, the low side reads the

evaporator pressure and the high side reads the discharge

pressure. I had the same problem while checking the ac on a

2000 gmc 1500 z71 4x4. other things that could cause this are

to much oil in the ac compressor or not enough freon in the

system. an overcharged system , the best way to check it is with

a gauge that way you can tell if you have the right amount of freon

and oil in the system. plus the gauge set will let you know the

extent of your problem. good luck!

This could be a few things.
1- Check fuses for A/C
2- Check to make sure the unit has freon
You have a switch on the accumulator that is called a low pressure shut off switch. It prevents the compressor from running when the freon is low. I would suggest if it is not a blown fuse, have a certified A/C repair garage take a look at this.

Purchase a new mode selector switch for it at any Chevy dealerships parts department

is the air coming out of the vents cold?

change the fluid and filter if u havent already if that dont fix problem have trans. checked out u didnt say how many miles are on the vehicle?

Check your fuses.

it appears the shutter that blocks off the heater core is no opening fully. It is vacuum operated and mY HAVE A HOSE DISCONNECTED. The vacuum HOSE AND actuating control is accessible under the dash on the firewall passenger side. Good luck

Sounds like the blend door in the ducting has failed, this door is what controls the temp of the outlet air by blending heat and outside air or AC cooled air.

I had this problem in my pontiac grand prix. it was the dual zone door actuator.( I can't remember if that was the technical name) basically there is a door underneath and behind the dash, depending on how much cold or hot air you want there is an actuator that opens or closes the door.