u guys are right u cant turn on or off the catalytic converter.
basically how VW did it was the ECU can increase or decrease fuel consumption to either decrease or increase emissions via the nitrogen oxide trap.

Nitrogen oxide trap
This system traps nitrogen oxides, reducing toxic emissions. But the engine must regularly use more fuel to allow the trap to work. The car’s computer could save fuel by allowing more pollutants to pass through the exhaust system. Saving fuel is one potential reason that Volkswagen’s software could have been altered to make cars pollute more, according to researchers at the International Council on Clean Transportation.

This post has been edited by COOLPINK: Jan 16 2020, 05:41 PM