You might need a point in mechanic skill specifically (? I'm not sure)
A little off of the original topic of promotion:
You'll want a few points in both mechanic and logic anyway since, in later levels, having 0 points in those skills hinders your performance quite a bit. I'd maxed out mech and believed the lack of logic would be ok but was wrong. I failed diagnosing things so much that I barely made any robots. All of my time was spent trying again. I have heard the same thing happens to people who have logic but zero in mech. (Fish after fish?)
Happily it only took a few points of the wrong skill (logic, in my case) to make a huge difference. I took 5 for good measure and had the same success rate as someone with maxed out logic.