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> This sentence from the article makes it sound like the software made the emissions be under the limit all the time.

The use of the word “pass” leads me to assume that it’s referring to levels when being tested

> ...would PASS emissions limits imposed by the US.

(Emphasis added by me)

It's worded poorly and should say "would pass emissions TESTS...", I think. What does it even mean to pass a limit?

Not to exceed it. Aka your limit is 500ppm, passing the limit means you did not exceed the limit.

That's not English.