When the OBD system was created auto manufactures (via dealership trade organizations that are extremely powerful in some states due to the large sums of taxes paid on new vehicles) pushed for state regulations that don't allow cars to be used that have the check engine light on. Often the sensor not functioning has no effect on the use or safety of the car (also the sensors themselves are designed to be needlessly difficult to replace). In effect causing the use of older cars to not be cost effective. In my state emissions are measured at the tailpipe, you can be well within standards but fail simply because a sensor reports that it needs to be changed.

This is essentially a highly regressive tax scheme that subsidises auto makers and dealerships at the disproportionate expense of poorer working Americans. It gives auto manufactures a "replace car" light that they can activate and have enforced by law and thus, planned obsolescence.

As someone who grew up poor and has spent time not having access to food in order to replace a useless sensor in a family vehicle (or paying a fine related to not having a valid inspection sticker) and knowing many other families who have been in the same situation. I have seen first hand the issues with this policy.