Airconditioning problem

The car is a 1998 Buick LeSabre Limited, 3800, etc. It has the electrically operated air conditioner control system (as opposed to the vacuum one) and does not
have a cabin filter.. Both the passenger and the driver can control the temperature in their respective zones...or could.
Now, we are unable to get cold air to come out of the dash. We can hear the fan blowing like clappers, and some air will come out the floor vents and some out the window vents, but zippo out of the dash when set to that selection. The air that does come out is cold, so this is apparently a fan/diverter problem rather than a sealed system issue.
The dammittohell computer was changed a couple of years ago for about $600.
Anybody have a clue what this might be? We are going to sell the car and would like it to be working properly for the next owner, BUT we dont want to put more into it than it is worth.
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No simple answer here, the door actuator could be an issue or commands from the control panel,

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Thanks, Shep... About what I expected to hear, frankly. I can dig under the dash a bit, and take it to one of the few reputable and competent mechanics here if I cant find the problem.
I would much prefer to sell the car "as is", with full disclosure to the buyer, but my wife is a person of unusual ethic, and wants it fixed before we sell it;>)
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Solved, at least for now. The vacuum control module was apparently not commanding the diverter doors, was stuck. After it was freed up, the system is working as it should.
I realize that I may have to replace that module in the future, and it is an expensive item, but for now, all is cool.
Thanks for all comments.
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