What effect does a malfunctioning thermostat have on emissions?

A malfunctioning thermostat can lead to increased hydrocarbon (HC) and carbon monoxide (CO) emissions due to the disruption of the engine's optimal operating temperature. When the thermostat is stuck open, the engine may run cooler than intended, which can hinder the combustion process. A cooler engine can cause incomplete combustion, resulting in higher emissions of unburned hydrocarbons and carbon monoxide as the fuel does not burn efficiently. Furthermore, when the combustion temperature is lower, it does not facilitate complete oxidation of the fuel, contributing to these increased emission levels.

This issue stands in contrast to scenarios where the engine operates at the correct temperature, allowing for optimal combustion, thus minimizing harmful emissions. Therefore, a malfunctioning thermostat primarily contributes to a rise in HC and CO emissions.