Officials in the greater Mexico City area have been unable to stop corruption at vehicle emissions testing centers, so they are now going to inspect most cars by computer.
Inspection stations currently measure tailpipe emissions, but inspectors have been found switching cars and results to help dirty vehicles pass.
Now, sensors will be hooked to car's own onboard computers and the readings will be relayed to a central processing station.
About two-thirds of the 5.4 million vehicles in the metropolitan area are 2006 or later models, which have onboard computers suitable for testing.
The other one-third can still be tested by the tailpipe sensors.
The new rules announced Tuesday will exempt cars made in 2016 and later from twice-yearly emissions checks for as long as four years.