See Figures 1, 2 and 3
The most conspicuous emission control is the catalytic converter. Its function is to combine unburned hydrocarbons (HC) and carbon monoxide (CO) with oxygen to produce carbon dioxide (CO 2 ). It also breaks down oxides of nitrogen (NOx). The catalyst operates in a very narrow range of air/fuel concentration. The oxygen sensor in the exhaust system converts the unused oxygen concentration to a voltage which the ECU reads. The ECU constantly adjusts engine air/fuel ratio to provide the proper gas feed to the catalyst.
This is a totally passive device, in that there are no actuators or sensors. It has no influence on the operation of the engine, unless it has melted and causes high exhaust back pressure. If this is suspected, the catalyst can be removed for inspection. By looking into the catalyst inlet, it should be possible to see through almost all of the passages in the honey comb pattern ceramic blocks. If there is any melting which may cause high back pressure, it will be quite obvious. On some vehicles, the catalyst is mounted directly to the exhaust manifold, on others it is farther down stream. On all vehicles, the catalytic converter is before the muffler.