What could cause a vehicle to fail for high NOx emissions with a P0128 trouble code?

A high NOx emission failure in conjunction with a P0128 trouble code often points toward the engine not reaching its optimal operating temperature, which is related to the thermostat's functioning. A stuck open thermostat prevents the engine from warming up sufficiently, leading to incomplete combustion and elevated levels of NOx emissions.

When the engine is not at the ideal temperature, it may not operate efficiently, and the combustion process can become less effective. As a result, nitrogen oxides—produced at higher temperatures—can accumulate and lead to high NOx emissions, prompting a failure during emissions testing.

Understanding the role of the thermostat in regulating engine temperature is crucial for diagnosing emissions-related issues, which is why a malfunction in this component links directly to the observed problem. Other choices do not directly relate to the P0128 code or the high NOx issue as clearly as a thermostat malfunction does.