In California, there is mandatory smog testing.  It isn’t really meaningful, the worst polluters are actually exempt and the system has even led to a special class of smog testing company – test only shops – which means they won’t be able to fix the problem so it will pass the test, but drivers may still be required by law to use one by law but then take their car somewhere else to get fixed if it does not pass

Smog testing companies know these things are dumb and a bad test result can be caused by various issues and the employees at those companies are in the people business, not the arbitrary law business, so they sometimes take pity on the customer because they would like for people to return.

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