Just replicating the reply I PM'ed to you, in case someone else may find it useful:
Actually, besides the additional sensors, a digital, dual zone unit has also a different recirculation box. Not only because it uses a potentiometer-controlled servo motor (manual units use a motor which is essentially the same, but without a potentiometer), but also because the flap mechanism is different. So, just replacing the motor with one with the potentiometer wouldn't solve the problem.
In a manual A/C box, the flap mechanism only moves from open to close, or from close to open, upon receiving a voltage, or a reversed voltage, from the control panel. The motor only moves abou 1/4 of a turn in order to make either operation. In the automatic box, the flap moves from recirculation to fresh air then back to recirculation while the motor is turning in one direction, and the motor moves about 3/4 of a turn. The first part of the flap movement (recirculation to fresh air) corresponds to a longer movement of the motor, probably because they need a more precise adjustment for the flap since that's the part where the A/C control module regulates the fresh-air/recirculated mix according to the air quality sensor. In the final part of the movement, the flap moves from fresh-air to recirculation in a short quick move.
Since your motor doesn't have a potentiometer to inform the control module where the flap is positioned, the control module doesn't know if the flap is open or closed or anywhere in a middle positio, then it generates a "potentiometer open or short to plus" internal error. It's probably trying to move the motor with a default polarity every time you turn on the car, then it moves the flap to recirculation, but it doesn't know what it's doing.
Simply replacing the potentiometer with one with a potentiometer won't solve the problem because, as I mentioned before, the flap mechanism is different. The control module would receive a much narrower range of movement from the potentiometer and would signal an "actuator stuck" error, It seems that the only way to have a fully functional recirculation flap, compatible with the automatic A/C control module, would be replacing the recirculation mechanism. That's not simple. I bought one to study the case, but installing it requires removing the A/C box, which requires removing the dashboard, which requires removing almost everything, so I didn't find it worth the effort.
What I did to solve that issue was installing the motor with a potentiometer in the recirculation box and building a small electronic circuit to convert the signals sent by the potentiometer so it seems to the control module that the movement range is as wide as the expected from a proper recirculation flap mechanism. It's a tiny 1.5 x 3.0 cm board with a dual op amp IC which is powered by the motor potentiometer wiring itself and I mounted it as tap on the wiring loom, so it didn't even require a case. I can send you the plans for the circuit if you have the electronics skills to build the cuircuit board (they are poorly handwritten though, sorry). Anyway, it's by no means a perfect solution since my recirculation flap only opens a small aperture when I press fresh air, never opens fresh air fully, after all it's just an amplification of the signal sent by the potentiometer (and the contol module tries to open/close the flap only within the final range of the mechanism, which is very narrow). I'm working on a new circuit that makes a non-linear conversion of the potentiometer signals so I can have a wider movement from the flap fit into the narrow range expected by the control module, but I'm not having much time to work on it currently.
Anyway, sorry for not having a simple easy solution for that issue. I think you could try to invert the motor wires so you have the flap set to fresh air when you turn on the car, but that may invert (or may not invert) the function of the fresh/recirc button, but I think it's worth a try. Please let me know if you find any more useful info on the issue with retrofit A/C untits elsewhere.
