A vehicle shows high HC and CO levels before and after the catalytic converter. What does this likely indicate?

When a vehicle exhibits high levels of hydrocarbons (HC) and carbon monoxide (CO) both before and after the catalytic converter, it indicates that the catalytic converter is not functioning as intended. The primary role of the catalytic converter is to reduce harmful emissions by converting hydrocarbons and carbon monoxide into less harmful substances such as carbon dioxide and water vapor.

If the converter were performing well, there would typically be a noticeable reduction in HC and CO levels from the inlet to the outlet. High readings on both sides suggest that the catalyst is either not active, damaged, or has deteriorated to the point of ineffective operation. This can be due to physical damage, contamination, or a failure in the catalyst's chemical structure.

Other options suggest scenarios that might cause emissions issues, such as excessive fuel mixtures or a lean air-fuel mixture, but these would usually manifest differently in terms of the emissions profile seen both before and after the catalytic converter. The consistent high levels across both measurements are a clear sign of catalytic converter failure.