Maybe there's something I missed, but these two skills escape me. Are they just the same thing with different ways in? How are they different?

Slightly related: what happened to the Demolitions skill?

Tech-Use is for operating tech (like repairing a broken Vox), Common Lore (Tech) is for knowing stuff about Tech, to quote the book:
Tech: An understanding of simple litanies and rituals to sooth and appease machine spirits.

There's a bit of overlap between the two since Tech-Use can also be used to figure out how unknown tech works.

Demolitions is a special use of tech-use (see page 91).

Musclewizard said:

That is… confusing. Appeasing the machine spirits would make the machine work, running on 40k logic. Thanks for the help, though.

And what a sorry fate for Demolitions. I'm all for skill consolidation, but that one should've stayed.

Plushy said:

Well the problem is inherent in the Tech-Use skill since its an extremely catch-all type of skill. By removing all knowledge related tests from Tech-Use the skill would become a bit more focused. I'm quite sure this has been discussed elsewhere for quite some time already though.

I do usually see it as a difference between knowledge and practical application. For example:

Tech-Use: I can use a computer and fix one

Common Lore (Tech): I know how a computer works

Just because you know one, doesn't automatically mean you know the other - similar to the change Muscle suggested, basically.

Common Lores are knowledge one gains just being around or working with things and organisations. Common Lore (My Office) would include stuff like where the kettle and tea are, the hidden hierarchy which may be different from the official one ("Well, technically they both work together, but generally x deals with this stuff and y deals with the other. Oh, and don't piss of z. She may only be a secretary but she can make your life hell"), which computer has a funny cable which means you have to twiddle it around a bit to make it work. It could also include more practical knowledge when that is directly related to the experience you have. So, you might have a basic knowledge of some program where you have used it on an everyday basis, but it wouldn't help you sort out a problem you had never come across before.

Basically, with Common Lore (Tech) you will probably know the common stuff (like the major litanies), and problems they have learnt to fix by rote (though this may overlap with Trade (Technomat or whatever it is), or very obvious problems ("Your weapon's  Machine Spirit is sulking because you have failed to feed it with a lasgun charge pack. This one is fully drained"). However, anything unknown, or anything that would need real investigation, they would have little chance of doing anything about.

Basically it would also only really apply if the simple knowledge of something would be enough. Tech Use is broader, and much more practical. If the item seems not to be working because someone failed to incorrectly press the activation rune, then Common Lore (Tech) might be enough. If there was a significant malfunction or it needed repairs then you would need Tech Use (and possibly Scholastic or Forbidden Lore (Tech) in some circumstances).

Common Lore (Tech) is the skill that I could see lots of non-Mechanicus types learning, for dealing with the simple technical knowledge they may need on a day to day level.

Or at least this is how I would see it…

borithan said:

That makes a ton of sense, actually. Thank you!

If you're really in a jiff about whether CL (Tech) applies, you can just add a -20 penalty to the test.

MILLANDSON said:

We wish.

But that would make sense, so that's not how it is :(

Take a look at repairing vehicles (p. 211ff)

Now there's stuff I want changed! Preferbly to use Tech-Use and maybe have the other skills mentioned as requisites,

Well, isn't common lore (Tech) one of the basic repair skill needed to fix vehicles?  Not major repairs, like re-doing an engine, but light repairs can be done with only common Lore (Tech)