Won't change vents only blows on head vents - 2008 Chevrolet Silverado 3500HD 2WD

HAVC Vent Door/Motor problem
Need a service manual or repair shop

I think the flaps are sticking

your heater core is possibly stopped up if there has been a radiator leak and stop leak has been poured its definetly clogged or could be a faulty thermostat

That car has 'blend door motors' that will switch the air flow. You are probably looking a problem with the control head or 1 of the motors that switch the vents and air flow.

The mode doors are operated by vacuum. Check the vacuum supply line to verify it is connected to the intake manifold. If that looks good, remove the control panel to see if vacuum is coming to the mode switch. Then check to see if the switch is transferring vacuum to the actuator lines. Here is the panel removal procedure:

1. Disconnect the negative battery cable.
2. Remove the necessary bezel(s) in order to gain access to the control head.
3. Remove the screws that fasten the control head to the instrument panel.
4. Pull the unit out to troubleshoot.

Possible that the vent control is blocked or is being turned off. you will need to check this out or it can be fault within the blower, the vent control or a physical block.
The loss of cooling can be due to many reasons and it can be related to clogged evaporator, faulty, compressor, low gas or faulty fan/condensor.
I think you must take consensus before any part is changed, check all the controls, check fan or it might need a resetting of the blower/evaporator unit.

The vacuum control head is broken, and defaulted to the defrost setting.

The heater blend door actuator (motor) is faulty.Replace actuator

check main vent on off switch.

Sounds like you have a supply vacuum problem. When you lose supply vacuum to the control head it defaults to defrost.