What result indicates the catalytic converter is not reducing NOx properly?

A catalytic converter's primary function is to reduce harmful emissions, including nitrogen oxides (NOx). When evaluating its performance, high levels of NOx after the catalytic converter indicate that it is not effectively converting these emissions into less harmful substances, such as nitrogen and oxygen, or nitrogen and water vapor.

For instance, if NOx levels are elevated after passing through the catalytic converter, it suggests that the converter is either malfunctioning or not properly treating the exhaust gases. This could be due to various factors such as a faulty catalyst, issues with the engine's air-fuel mixture, or other related problems.

In contrast, low NOx levels would indicate that the converter is functioning as intended. High CO₂ levels after the converter might occur from complete combustion processes rather than indicating malfunctioning efforts to reduce NOx specifically. Similarly, high hydrocarbon levels before the converter would not directly reflect its ability to manage NOx emissions. Hence, the indication of high NOx levels after the catalytic converter is a clear sign that it is not performing its intended function properly.