Motor vehicle manufacturers have been required to meet increasingly stringent new vehicle pollution standards. But vehicles that are poorly maintained or that have malfunctioning emission controls often exceed those standards. Even minor malfunctions can increase emissions while major malfunctions can cause emissions to skyrocket. The average car on the road emits three to four times more pollution than standards allow for new cars.

The percentage of dirty vehicles increases with age. Roughly 30 percent of five-year-old cars emit excessive pollution. At seven years old, the average age of passenger cars in the United States, 55 percent of vehicles are high emitters.

Overall, 10 to 30 percent of vehicles cause the bulk of the problem. Unfortunately, it is not always obvious which vehicles fall into this category, as the emissions themselves may not be noticeable and emission control malfunctions do not necessarily affect vehicle driveability. Effective I/M programs can identify these problem vehicles and assure their repair.

(From EPA)