I have been told by some friends that it is heresy or some kind of crime for any non-mechanicus to even attempt to build or fix any tech at all. Is this true? I don't see it anywhere that says only Mechanicus are allowed to perform repair rites.

That is untrue. Hive world manufacturing wouldn't work if it took the Mechanicus to perform all repairs.

This is one of those things that may be written down somewhere but nobody actually follows. Kind of like how the space marine holy text says they're to spend 20-odd hours of the day praying to the Emperor. Nobody actually does it. The Mechanicus will love to tell you about how they are the only ones, who through centuries of ritual memorization, are the only ones qualified to repair technology (all of which is sacred, blah blah blah), but in reality it doesn't take a genius to swap out an air filter or polish a capacitor contact. That rule exists not to punish tinkerers, but to maintain the Mechanicus' monopoly on technology so that Mars and Terra get along nicely. If the Imperium started seeing to its own technology needs it wouldn't need Mars, and there was already a war about that.

Ultimately it's your game and you can play it however you want, though.

edit: also this isn't a Rules question.

Well I was asking if it was a game rule or not, but okay.

Oh, well no, there aren't any rules about being a member of certain faction in order to be allowed to perform a certain action.

This is one of those things that may be written down somewhere but nobody actually follows. Kind of like how the space marine holy text says they're to spend 20-odd hours of the day praying to the Emperor. Nobody actually does it. The Mechanicus will love to tell you about how they are the only ones, who through centuries of ritual memorization, are the only ones qualified to repair technology (all of which is sacred, blah blah blah), but in reality it doesn't take a genius to swap out an air filter or polish a capacitor contact. That rule exists not to punish tinkerers, but to maintain the Mechanicus' monopoly on technology so that Mars and Terra get along nicely. If the Imperium started seeing to its own technology needs it wouldn't need Mars, and there was already a war about that.

Ultimately it's your game and you can play it however you want, though.

edit: also this isn't a Rules question.

!!! Heresy!

There's also fluff to suggest that Tech-priest do train a number of Lay-Techs in how to perform particular rites (procedures). Things like how to change the oil on the car or other routine tasks. If something goes wrong or looks weird, then they call in the Tech Priest.

There's also fluff to suggest that Tech-priest do train a number of Lay-Techs in how to perform particular rites (procedures). Things like how to change the oil on the car or other routine tasks. If something goes wrong or looks weird, then they call in the Tech Priest.

That is Trade (Technomat) . You learn to use tech or do upkeep on tech, but not by learning how it works. You just learn what to do to get what effect. If I press this button with the correct prayer code, the machine will do this

I think it is more an issue of resources, even if you have specialized workshops you'll still need to either try trial and error a lot of times or have a blueprint, and who has the most blueprints? Mechanicus of course.

The mechanicus controls all the technomats and servitors indirectly though overseers who have the mechanicus blessing. I'd say if someone is messing with tech, and a tech-priest can't trace that person to another tech-priest, then the mechanicus will start bringing the hammer down on those people for breaking the Imperial bureaucratic hierarchy. (Adeptus Mechanicus is an official branch of the Imperium that the others must respect). Any persons acting without their blessing will quickly find themselves unable to get supplies or support, and may be investigated for tech heresy if the operation is large enough.

If there was a whole planet with no mechanicus presence, it would be very heretical indeed, and might have to be reconquered.

Even the Adeptus Astartes and Inquisition's engineers are trained by and are actively members of the Adeptus Mechanicus.

That said, there are no rules regarding this.

Personally, I'd say it pretty much boils down to the "this is what it says on paper" things as cps mentioned. The Necromunda material mentions entire workshops and factories run by gangs as even the Arbites generally don't dare go down there. Laws are only worth as much as you can enforce them, which will differ from planet to planet, and even on a single world from location to location.

At the same time, non-Mechanicus adepta like the Space Marines or the Sororitas can have personnel "sanctioned" to maintain technology, and as Quicksilver mentioned there can also be civilian licensees.

Ultimately, it also depends a lot on one's preferred interpretation of the setting and the sources you follow. But as far as this game's rules are concerned ... no, there's nothing like that. If you are able to buy the Skill, you can use it. The GM may still rule some technology as being too unfamiliar for your character to understand without specialist knowledge, though...