if the BOV results in a failure of the turbo or related components, then yes, the BOV killed the warranty. If your AC compressor stops working and you have a BOV, the BOV is not the cause and the AC is still covered. Installing a single aftermarket part doesn't void the warranty for the entire car. If you install an aftermarket part that leads to the failure of a factory part, or if the installation of the aftermarket part resulted in the removal and re-installation of a factory part and that failed, the aftermarket part will void the warranty for the failed parts. I have been modifying new cars since 2000 and never had any warranty issues. I have been asked to return some parts to stock configuration during a troubleshooting just to make sure the aftermarket parts didn't cause anything to go wrong. That was on my 2000 Focus in which it had a very strange electrical issue that took years to solve. Turned out to be a bad ground and had nothing to do with the aftermarket parts.