I've had a broken, out of warranty Xbox 360 lying around for some time. Microsoft want €135 to repair it, which isn't worth it. A friend's friend does repairs for €40. Would this repair cause MS to recognise it as a modded console (ie is the check for nodded consoles a check for unauthorised components or just a check for opening it up)? I can do without Xbox Live on it, as I still have my main 360, but it'd be handy if two could play the same game online together at once.
Opening your Xbox 360 is not the same thing as modding it.
Modding, short for modification, refers to the act of making changes to your system in order to favorably (and often illegally) circumvent a blocking mechanism. For example, you might mod your Xbox 360 so that it can play burned or downloaded games without permission. This will get you banned from Xbox Live and disowned by Microsoft.
Opening the system, on the other hand, will only void the warranty. It's not illegal and you won't be penalized for it, especially since your warranty has already expired. However, Microsoft may refuse to officially repair your system, in the event that your friend is unable to revive it.
Edit: There's no way Microsoft can detect from afar if you've opened your box and fixed it yourself. They don't want you doing your own repairs because a) you might turn around and blame them after you've made things worse, and b) because they make money from doing the repairs exclusively.
Short Answer: Your console will not be recognized as modded.
Microsoft can, and will, detect it.
The xbox360 has a logging system, and sends it's logs to MS.
It logs failed boots, boots without drives attached, fan voltage changes, drive serial number changes, etc.
Authorized repair centers have a way of correcting the logs.
If you boot even once without the drive attached, the console logs it, it's sent to MS, and you get added to the next ban wave (they assume you flashed the firmware).
If the logs show repeated RROD, followed by successful boots, they can flag you for that.
Oh, and if you change the drive without reflashing the serial back JUST right. Logged and banned.
This whole unauthorized repair business is to catch stealth modders.
That said, you may or may not get banned. Booting without the drive attached nearly always triggers it, juicing up the fan definitely triggers it, while fixing RRODs doesn't always trigger it. Most likely depends on how many RRODs in a row you got before it was fixed.
In all cases, this is a delayed ban, added to the next ban wave. MS wants you to get used to playing online so you will buy a new 360 to replace the banned one.