Hello to all contributors and 850 owners,

I have been away for a few weeks but owe you all a conclusion to this long exasperating tale. First of all you should know that I have had SUCCESS. As I said before I had swapped out a couple of resistors from the boneyard and tried a couple fans and then swapped out the climate control box... no combination worked. Codes, particularly the 419 code stayed the same.
Back to the boneyard and I asked the owner (nice guy) to find an 850 where the ECC system is working and then I would try to replace the parts one by one... started with the climate control box as mentioned above and no difference...tried that with the (new) system fan... no difference. Then I tried swapping out the resistor ... this was the 4th or 5th one I had tried and everything started up. Hooray... swapped back my own climate control box.. all good... swapped out my own fan ... all good. Conclusion is: as codes, Klaus and others had indicated that it must be the resistor.
But here is the rub... this is where you need to pay attention. I checked the numbers on the back of this particular resistor and they were identical to the original... none of the others matched. It just so happens that this unit came out of another '96 and it worked. The mechanic, who works on nothing but Volvos said that he heard some time ago (and now remembered) that you need to find a resistor with matching numbers. He wasn't sure but thought that may have been a particular quirkiness of the '96 year only but was not sure. Let me tell you that they all look and assemble exactly the same. Now I'm not saying that I know for sure but if ANY of you run into this same issue, and I have heard of people returning resistors, even new or rebuilt ones because they didn't work... START with one that has the same part number as the original. It just may save you a whole lot of grief. Thanks to all contributors and particularly Klaus for insights and code reader build plans etc. I still have 124 and 126 so ECC Auto setting is a little sketchy... but I have HEAT. Thanks to all..