It is a bigger disaster than what it is reported, at the beginning a lot of persons bought old cars not just one but some times two with a plate ending in a different number from either the DF on the Estado de Mexico or neighboring states, this only added to the smog and congestion and did not relieve the problem, latter they started the smog check and this became ramant corruption on the part of the smog checking stations.
There was an instance where one of the TV stations purchased a brand new vehicle, and immediately took it to be smoged or "verificado", the vehicle of course did not pass, they wanted a mordida for it to pass known as a "brinquito", this led to an investigation and some of the stations to be closed, today not much has changed the verificacion still asks for a brinquito if they see that you are naive enough to tolerate this.
In reality it is very simple to understand for Canadian and USA plated vehicles, depending on the last number of the license plate is the day of the week that the vehicle does not circulate plus Saturdays, all license plates ending with a letter they do not circulate on Friday and Saturday and the hours that foreign plated bvehicles are forbiden to circulate, in the days that you can are from 05:00 AM to 11:00 AM.
Forget about the color decals and the "O" and "OO" decals, those only apply to vehicles that are registered in the Federal District and the Estado de Mexico, no foreign registered vehicle can apply for this.
I have in previous post given the numbers and days that correspond to the hoy no circula for foreign plated vehicles.
navegator