Ok so here is what I know and how I solved the issue. GMPX thanks for your help, you were on the right track as I have looked here as well regarding air conditioning. We hadn't wired in a pressure switch because we have other vehicles running without it. But A/C has been in the back of my mind as it was a DTC. What I didn't realise or maybe understand is, I thought turning the DTC's ti not reported etc just stopped them logging when reading the fault code. But apparently having them Not Reported actually stops the PCM reacting to the fault as well. I was unaware of this fact. I thought all the other areas of programing , Fan Controls, VATS, Engine stuff etc was where it all happens. Well after weeks of playing and asking tuners and other programmers who had no idea why, the problem is now sorted.