I think one of my first posts was about "smog" pumps.
You would be amazed the number of technicians that think a smog pump recirculates exhaust to clean it. Actually most think the EGR does this. Neither does.
The "smog", actually A.I.R. (air injection reaction) pump adds air to the dirty exhaust in the presence of heat which causes part of the remaining incomplete combustion to combine. This was the first form of smog control coming out in 1968. Some cars even created a chamber (reaction chambers) at the manfold to enhance the burning process.
In the modern engine the process is used for the same reason and the additional reason of heating the cat. The air injection must stop before the O2 sensor warms as if there is still air injected the car will be viewed as lean by the management system and fuel will be added.
The air injection event is very short on a modern engine; and only when cold.
Bosch Master, ASE Master, L1
33 years MB technician