At least I think that's what the nano said. Right hand blend motor on a friends 38 btw, forced it with nano and the percentages don't alter but fine testing the left hand blend at all percentages. We can hear the motor working though on both. Removing it and testing the motor with a 9v battery and all is good, no gears stripped either.
The board looks good too and as the nano doesn't give a potometer fault I'm assuming that is good. The cable going to the multiplug on the back of the hvac looks perfect too. He does have a spare set of unknown motors complete off a scrapper which we will fit later today(just the one, cut, solder, shrink wrap). Hopefully that will fix it and get the book off unless there is something we have overlooked! We are both a bit green on 38s but learning slowly lol!

The feedback pot isn't sending a reading to the HEVAC. When you force it to move from one setting to another using the Nanocom, you should see the percentage change as it moves. If it isn't, then the HEVAC doesn't know what position it is in so flags the error and doesn't try to move it again. The Nanocom error isn't really of much help, but it does at least tell you which motor is causing the error and you can then confirm by doing as you have and looking for a change from the feedback. On initial switch on, the HEVAC drives the blend motors over their range to check for correct feedback. If it doesn't see a change it stops at one end of the travel or the other. Sods law says it will stop at full heat in summer and full cold in winter. You can sometimes get it working again by giving it a squirt of switch cleaner and working it back and forth a few times. If you take the gear off to work the pot, make sure when you put it back the arrows line up.

Cheers Richard, we replaced that pot last year? and I replaced one on my car maybe the year before. Mine failed just recently again and it looks like his has as well, both pots came from the same source but were free from a fellow p38 owner who had sold up. I suspect they are cheap rubbish and we had to fit them 90 degrees out from the arrow to get the full sweep too. Have you got a source for decent replacements at all?
I fixed mine by swopping the cheap pot for a used unknown but original one and that's been fine for a few weeks now.

The ones I use are from Mouser Electronics, Mouser part number 531-PT15GV02103A22ES (https://www.mouser.co.uk/ProductDetail/Amphenol-Piher/PT15GV02-103A2020-E-S?qs=DPoM0jnrROVsaSbOUHWyTQ%3D%3D). One problem I found and it may be that you have the same, is that they are not quite as tall as the original ones, so it is possible for the gear to ride up the spindle so it no longer meshes. My solution for this is to put a couple of small plastic washers (from some bits left over from assembling a desktop computer for my step-daughter) on the spindle so it can't rise up and become disconnected.

Okey dokey, do the arrows line up as original?
Contact cleaner didn't work unfortunately, we've tested it with a dmm as that's all we've got and it's showing a noticeable dead spot, still giving a reading but it doesn't alter in that 'dead spot'.

Yes, the arrows line up as they should with those pots. The dead spot will be what is causing it to fail the self test.

I had a spare pot so we tested that with the dmm and got consistent readings so soldered that in and.....its still the same! Percentages on the nano do not alter at all, bit of a let down. We can hear the motors and flaps move even when using the temp button on the hvac, same with the nano.
Had to knock it on the head for today. I'm thinking maybe the board that the motor etc is soldered to may be faulty, the cable is damaged somewhere, the hvac multiplug has an issue or even the hvac has a fault?
What would be the next logical move?
BTW, the fault won't clear with nano. Tried cycling the ignition too.