JARblue

Aftermarket mods are not covered by warranty, although I'm not exactly sure about putting a GT-R start button on a Z (I would guess they still won't cover it). If the dealer can claim that your aftermarket part or the install of an aftermarket part caused the problem, they won't warranty stock parts that fail because of it. So unless your start button causes something else to fail (highly unlikely) or fails itself, I wouldn't worry about it.

If you're that concerned about it, just keep the stock button around and reinstall it if you have to take your car to a dealer.

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