What kinds of tuning can ruin a car's ability to pass emissions testing?

Question

Suppose I want to tune a car for performance, like doing an ECU re-map for a Toyota Supra.
Could this affect the car's ability to pass emissions testing? If so, how?
And in general, what kinds of tunes will / won't jeopardize a car's ability to pass emissions testing?

Solar Mike · Answer

Performance tuning changes the fuel delivery and other things like the ignition timing to develop more power.
This is fine, but when certain countries set emission standards of x at y rpm then due to the changes the limits can be exceeded. This is one reason why older vehicles can fail emissions as worn components cause drift on the target values so the standards are no longer met.

What kinds of tuning can ruin a car's ability to pass emissions testing?

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