Didnt have the patience to read through all this, sorry...whats the serial number on your Hevac head unit? I suspect it's an early one that has the snow flake issue and that using a later one may help. I had the same issue on my second p38....changed head unit, problem solved

The early one looked at the amount of current being drawn by the compressor clutch (as it was driven directly from the HEVAC), the later one didn't. On a later one the HEVAC only powers the relay so doesn't check the amount of current being drawn. That is why the mod requires the resistor so it draws sufficient current to not flag it as a fault but on a later car the resistor isn't needed.

No, this is a lesser known issue - the earlier head units also had a tendency to stay at -40/ snowflake mode after start up. Can't remember where i saw that... rr.net probably, but as soon as i changed the HU for a later serial number the problem was solved. I suppose the serial number thing could be a complete red herring, but the change in HU certainly solved the issue for me

Found similar on my bench test when the dummy ambient sensor was not connected. Even after connecting, the HEVAC needed faults clearing & power cycle to get the temp to update. According to RAVE later HEVAC show an initial reading on power up, but don't update until the vehicle reaches 15mph.

Unless someone disassembles the firmware, we will probably never know how the HECAV combines all the inputs to decide where to adjust the flaps !!