It was NOx, and they didn't self certify, they cheated the tests.

Car companies operate their own testing units at their factories. I don’t see how this is different from self certification.

Ex: “Today in Tokyo, Nissan joined the increasing list of automakers who have admitted to falsifying emissions and/or fuel economy figures. The company said it had uncovered falsified data from car exhaust emissions tests at most of its plants based in Japan.” https://www.forbes.com/sites/peterlyon/2018/07/09/nissan-adm...

Trust but verify is common worldwide for this stuff. Otherwise governments would need to install and operate a lot of equipment at every factory worldwide.

True, but an entity that goes so far as to cheat on tests is hardly likely to honestly self-certify - it is further evidence that some non-trivial form of verification is necessary.

I am not, however, going to take any position on whether Boeing has done anything wrong until the accident analysis is complete.