If a car fails a smog check due to high NOx emissions and has an LTFT of +25%, what is the most likely cause?

In this scenario, the vehicle's failure due to high NOx emissions coupled with a Long-Term Fuel Trim (LTFT) reading of +25% suggests that the engine is running lean. A high LTFT indicates that the engine has been compensating for a long-term deficiency of fuel, which can occur when there isn’t enough fuel being sprayed into the combustion chamber for the amount of air entering it. This lean condition can lead to increased combustion temperatures, which in turn can elevate NOx emissions.

A clogged fuel injector could lead to insufficient fuel delivery, resulting in a lean mixture and higher combustion temperatures. The decreased fuel flow due to the restriction in the injector would explain the elevated LTFT as the engine control module attempts to compensate by adjusting the fuel trim upward.

While other choices could contribute to emission issues, such as a faulty oxygen sensor affecting fuel-air mixture readings, or excessive fuel pressure causing rich conditions, they do not directly align with the high NOx emissions in this context. Similarly, while a vacuum leak could lead to a lean condition as well, it wouldn't specifically explain the +25% LTFT as directly as a clogged fuel injector. This is because a vacuum leak may have other symptoms and behaviors at idle and part throttle that could differentiate