What would likely cause emissions to read normal (13.1 to 15.1) during operation?

In the context of engine performance and emissions, a situation where emissions readings are normal (typically between 13.1 to 15.1 parts of air to fuel ratio) suggests that the engine is operating efficiently, achieving adequate combustion. An engine that is too cold may indicate a number of conditions, including the engine being in open-loop operating mode, where fuel delivery calculations rely on pre-programmed values rather than live sensor feedback. This can initially lead to normal emissions as the engine attempts to warm up and may not have fully engaged all emission controls yet. As the engine warms up, however, it should reach a point where emissions stabilize within the desired range due to proper fuel atomization and combustion characteristics.

An engine that is running too cold might also exhibit higher than normal emissions under certain conditions, but initially reading normal could indicate the engine hasn’t reached full operating temperature where abnormal readings might begin to manifest. On the other hand, operating conditions such as a faulty throttle body or a failed Accelerator Pedal Position (APP) sensor typically lead to erratic engine behavior and can skew emissions readings, while an overheated engine might cause combustion and emissions inefficiencies, typically leading to abnormal readings.

Understanding how the operating temperature of the engine affects emissions can help