I have a 1999 chevy Lumina and the dash board vents keep going from the ac vents to the floor vents. need to find out where to check first

resrve tank and valves leaking

You may need to pull the battery cable to clear the memory codes.
Otherwise, the controller could have failed or something with a blend door. When the controller "sees" cool air it commands the blend door to change air vents.

Check vacumn line that runs from round accumilator under driver fender to check valve before it runs through fire wall. Runs under fender under battery . could be check valve or broken line. Had same problem it was broken line

NEED TO HAVE YOUR TEMP CONTROL UNIT CHECK FIRST. ONE OF THE VENT DOORS ARE NOT OPENING UP

The drain tube from the evaporator housing is most likely plugged up. With the vehicle in the air locate the drain tube on the passenger side of the firewall and using coat hanging or something similar carefully snake it through it Be careful as a lot of water may pour out and don't go crazy poking with the coat hanging as the evaporator core is at the other end of the tube.

you might have to replace it, its inside the dash board can be tricky but it can be done by you

I had same problem . There is a vacuum resovoir tank located behind the left front splash shield . The vacuum line behind the battery goes to it . I removed two 5/32 screws just in front of the tire that hold the splash shield to the fender , carefully flex the shield away from the fender and take a look . I found the vacuum line had come loose , plugged it back on and everything works . Hope this helps

Check the vacuum lines, starting at the firewall in front of the passenger side and trace backward.

You have a vacuum leak in the system that controls the vent doors. First check for cracks at the vacuum reservoir, then check the various vacuum hoses and vacuum motors for a leak.
Charlie

common problem, check under dash by the gas pedal if control arms are muving went selecting from ac to heat cuz this it is the control for the vents to go from ac to heat and to defrost.