I think most states will fail a car with the "check engine" light on. For cars in the northeast, keeping the CEL off gets more and more expensive every winter. I had a truck that was getting a new oxygen sensor (there were 6 on the car) every year from age 8 to 12. The truck drove fine with them malfunctioning, but it couldn't pass inspection. My current Jeep, I assume is going to need about $2K every year in maintenance to get it to pass inspection. And the only reason to do the maintenance is the inspection, because it also drives fine.