[thaeli]

For emissions related components, EPA rules do kick in though. While the current administration appears to have paused enforcement, their position for many years has been that running anything except factory approved firmware on an ECU or other emissions related computer constitutes a “defeat device” and is illegal for an on road vehicle subject to emissions controls. (Granted, in practice 99% of the reason anyone installs new firmware on their ECU, or switches to an aftermarket ECU, is for a “tune” that does affect emissions. I’m sure there is some edge case exception, but it’s very rare in on road engines.)

The alternative, and there are a very few tunes that have done this, is to prove to regulators that the tune does not negatively affect emissions in any way. In practice this is done by getting a CARB exception since they’re the ones actually checking for tunes.