I have a 92 S600 and I have also never been able to get the fault codes to show up, only the sensor values.
My problem was no heat comming out of the foot vents. I have through trial and error managed to get heat by closing the centre vents, pressing both buttons for lower heat and pressing the red button on the upper centre facia vents.
I don't know why this works. It's less than ideal, but without a diagram, and not being able to access the fault codes it's the best I could do.