[bradfa]

In states in the USA which perform emissions testing, many of them did not mandate it for diesel cars. For example, I owned a VW Jetta TDI and in New York (which has yearly emissions testing where an OBDII computer is mandated to be connected to gasoline powered cars in order to pass the yearly emissions inspection) and I was exempt from the emissions testing entirely.

A 3rd party sensor would be incredibly expensive for inspection stations to purchase as it would need to meter the air and fuel which enter the engine (assuming we aren't going to trust the car's computer which already knows these figures) as well as to measure the emissions out of the tail pipe. This is economically unrealistic to implement without a dramatic price increase in the cost of regular emissions testing.

Trusting the computer is the economical and realistically widely implementable solution. But yes, it has it's blind spots.