What condition is indicated by HC showing a large increase coupled with a large decrease in CO?

The scenario described—where hydrocarbons (HC) show a large increase while carbon monoxide (CO) shows a large decrease—indicates a very lean mixture condition.

When an engine runs lean, it means there is an excess of air compared to fuel in the combustion process. In this situation, the combustion process becomes less efficient, resulting in incomplete combustion. This incomplete combustion is primarily responsible for the increase in HC levels, as more unburned fuel escapes into the exhaust system.

Meanwhile, CO is a product of incomplete combustion as well. However, if an engine is running extremely lean, there isn’t enough fuel for the combustion process, which leads to a reduction in CO emissions. This reduction occurs because CO is produced when not all the fuel is fully oxidized, and with a lean mixture, there is insufficient fuel present to generate significant CO.

Thus, the large increase in HC paired with a large decrease in CO is a strong indicator that the engine is operating with a very lean mixture, supporting the selection of that answer.