Are you saying that on Max A/C, the center vents only produce warm or hot air and then ultimately cools down?? If this is the case, then you may not have enough of a charge or the compressor is hitting against high pressure. If you are getting air flow on econ and air flow on max A/C, but the temp is not right, I'd take a look at that as well as what the DOC said.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.