Which of the following may indicate an issue with a vehicle's emissions system?

Abnormal readings from the oxygen sensor are a significant indicator of potential issues within a vehicle's emissions system. The oxygen sensor monitors the amount of oxygen in the exhaust gases as they exit the engine, providing critical data that the engine control unit (ECU) uses to manage the air-fuel mixture for optimal combustion. If the readings are outside the normal range—such as consistently high or low—it suggests that the air-fuel mixture may be incorrect, leading to incomplete combustion, increased emissions, and potentially failing an emissions test.

In contrast, other options like clearing DTCs (Diagnostic Trouble Codes) and resetting monitors or stating that temperature levels are within the normal range do not directly indicate issues with the emissions system. Clearing DTCs may be part of routine maintenance or troubleshooting but does not necessarily reflect an active problem. Similarly, normal temperature levels do not provide insight into the functionality of the emissions system components, such as catalytic converters or fuel injectors.