What produces CO2: Engine or catalytic converter?

Don't confuse CO ( Carbon monoxide ) for CO2 ( Carbon Dioxide ).

CO is created from the incomplete burning of fuel. This happens in the engine.

The Catalytic converter tries to change the CO into CO2.

CO2 is completely harmless, it's what you exhale.

CO isn't. It's not bad enough to make everyone walk though :D

The combustion of Hydrocarbons with oxygen is source of energy for our fossil fuel engines.

This processes creates H2O & CO2. So it's fair to say that MOST CO2 is created by the engine.

Catalytic converters use the energy from some unburnt hydrocarbons to degrade other unwanted by products - most notable NO2 & CO.

So converters produce some CO2, but it's a small amount compared to the engine.

The by-product of burning fuel dissipates CO2. The catalytic converter heats up to a point that it burns up any excess gases not burnt in the combustion process of the engine. -- that's the idea anyway. Of course the Cats can't get 100% of the gases, but they do burn off some excess.

And remember, our trees and plants thrive on CO2!!!

It looks like Hey Dook is being given multiple thumbs down for a completely correct answer that was probably also the first given, shows the intellectual integrity of people on here.

As others have said, incomplete oxidation of the fuel generates both carbon dioxide and carbon monoxide in the exhaust. Carbon monoxide is quite poisonous and one of the purposes of the catalytic converter is to convert the carbon monoxide into carbon dioxide, which it does quite well. Just so that you don't get the wrong impression from other answers, carbon dioxide in high concentrations also kills people, but carbon monoxide is much more dangerous.

The engine produces CO2 and other gasses. Some of the other gasses are more harmful, so you have a catalytic converter which converts them to less-harmful CO2. So they both produce CO2.

BOTH.