2002 Voyager 3,3, AC/Heating problem

Got a 2002 Grand Voyager (as we name it in Europe), 3,3 AWD, with AC/Heating problems.
No matter the temperature setting, from 15C to 29, it will blow with same very hot air. AC working when setting the temp to LO.
Going from LO to 15 deg C, you´ll hear a "clunk", as if a door closes, and the warm air will start to flow through the vents.
Tried all the calibration possibilities known to me (ON+Recirc), and it will end with no faults, but to no good effect on the problem.

Does anyone know what the "ON+AC" mode does? Been in that mode - but do not know what it does, or if I´m supposed to do anything.

Annoying - and haven't got a clue, other than the "thump" I hear when, possibly, the blend door closes, might be a problem, since I would expect the blend to be more gradually invoked.

Any suggestions?

Do you have the Automatic Temperature Control, with digital indicators, like me? Mine also appears to act quirky. I replaced the AC compressor last summer and it doesn't blow cool enough, so my wife claims I am incompetent, or the Duracool refrigerant I used is the problem. What a way to talk to a guy with an MS in Mechanical Engineering. Anyway, I noticed that very cold air blows out of one of the middle vents, while much warmer air comes from the others. Also, the return line to the AC compressor is cold, so the AC evaporator must be cold. I conclude that the controller is allowing either hot air from the heater core or from outside (>38 C days are common here), even when we set both dual zones at max cold. I do have a spare ATC I got cheap on ebay a year ago (another problem I fixed without it), so I'll try that when I get time. No need to fuss over lack of AC for the next 6 months.

You may need to head to a shop and have the codes read. Not the typical powertrain codes everyone reads with a typical scanner. Doesn't come up too often but I think they fall under "Body" codes and could be "Communication" codes They start with a B or U rather than the typical P.