Same conclusion I came to after talking with both MB and Renntech, at length, when I was researching it in the past.

I was going to get a second stock ECU , but I have a good friend who is a MB shop supervisor ,and a good friend who is MB tech and both said if it comes down to a warranty issue ,especially broken drivetrain parts they will look at why 2 ECUs assigned to the car ,the mileage will be off etc , now not pursuing this any further and not looking into somehow syncing the mileage if possible I just decided not to mod any further then a tune while still under warranty .I guess anything can be manipulated nowadays.i have that open software update on my car but not bringing it in for that cause I know what’s gonna happen lol.

A certain, well known and reputable, tuning shop can clone your stock ECU into a second hand stock ECU, assuming you can get one... Then you tune the second ECU. But, like you said, if you actually needed extensive drivetrain warranty work, and they start digging into it, things aren't going to match up, and you will likely get denied. You can use this method for avoiding flags, but it's going to cost you $1,500+ minimum for the second ECU, from what I've seen on fleabay.

Bish-- agree, always caveat emptor, but if you talk to SAs and manager of a Mercedes dealership that is an authorized Renntech shop you will get a different story on how things will work with the bits and parts purchased from and installed by the dealership. Suspect your guy is not at one of those dealerships. We can agree that this cozy arrangement is much better then the OP rolling into a Merc dealership with an EC tune.

Off topic, but for others looking at a tune-- the fact that a car is tuned does not in and of itself void an engine warranty. The tune has to be the cause of or exacerbate the problem. Having been in the tuning game for a while (and hopefully not to jinx myself), the failure rate associated with tunes back to Kleemann, Renntech, EC and others is amazing de minimis. This is particularly true with Mercedes which continues to get away with the one motor/heavy detuning concept to price cars. Why should my GT now have more horsepower than a stock GT R after a simple ECU flash? (I know the GT R has some different motor internals, but come on AMG.)

I love these statements that a tune want have warranty issues if your engine blows lol. My dealer, an AMG performance center, used to install renntech. They will no longer install renntech tunes. Issues with MB and warranty claims. If you think that you bring a blown engine into service, that has been tuned, it isn’t going to raise serious red flags your dreaming. About 100k to have a dealer replace one of these engines. MB is all over this. If they deny a claim good luck. The attorney fees to fight MB will be almost as bad as the repair and you will most likely loose your case.

Please stop. If you mod your car under warranty accept the risks. Everyone stop crying and looking for ways to cheat the system. If you can’t afford the consequences don’t do it.

Not the case in regards to warranty. Another forum member (MuffinFlavored) has a 18 E63s with a tune. Came in for something else, had a contentious conversations with the SA about an unrelated repair and they reported the car as tuned to MB as payback.
No engine issue whatsoever. As a result, drivetrain warranty void and a likely more hassle during upcoming service and repair visits.
Cars do get flagged and once flagged you better not have a problem.
https://mbworld.org/forums/w213-amg/...-review-2.html

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This is a story, and a cautionary tale. I suffer from my knowledge of the law which sometimes gets in the way of good stories. We know that companies work hard to avoid large warranty claims in all walks of life. A tune on a car will never help to facilitate a warranty claim, agree. I'm simply pointing out that Merc or any other manufacturer or dealer must show that the tune caused or related to the need for the repairs before denying coverage. You can have a driveshaft problem (as some of the 2016 GTS's did) that has absolutely nothing to do with the ECU. A tune by itself does not void a car's warranty under federal law. That's it, all I was saying. Anybody that wants to wait until their warranty expires to feel safe and more secure can do so. It's OK.

They can do what ever they want, which is usually the case (no pun intended).

Yeah the driveshafts did have an issue in the early cars but if an increase in power torque due to a tune in any car experiencing such a failure of anything from the water pump to the rear axles can be a reason for the dealership to flag the car if it is found to have a aftermarket tune installed...

Porsche and Audi do this all the time and I know this first hand... Unless you have a stock ECU in the car when in the dealership you are just at their mercy when they check the OBD2 port for faults..
Doing a deep dive into the ECU by the dealership only happens when there is a factory upgrade or you are a complete bonehead and **** off the service manager...

Be nice and get nice in return... Hey what do I know