What happens to carbon monoxide (CO) levels when an emission system is tampered with?

When an emission system is tampered with, the carbon monoxide (CO) levels typically increase. This is due to the fact that emission control systems are designed specifically to reduce harmful emissions, including carbon monoxide, from the exhaust of combustion engines.

When these systems are altered or removed, the engine may not burn fuel as efficiently, leading to incomplete combustion. Incomplete combustion produces higher levels of carbon monoxide, which is a toxic gas that can pose serious health risks. Tampering with the system can compromise its ability to oxidize CO into less harmful gases, allowing more of it to be released into the atmosphere.

In contrast, if the emission system remained unchanged, the CO levels would likely stabilize at the manufacturer's regulated limits. A decrease or complete disappearance of CO would imply that the emission system is functioning optimally, which is typically not the case with tampering. Therefore, the only logical outcome of tampering with an emission system is an increase in CO levels, confirming the correctness of the selected answer.