What could a technician observe if the EGR valve is stuck closed?

If the EGR valve is stuck closed, a technician would primarily observe an increase in NOx emissions. This is because the EGR (Exhaust Gas Recirculation) system is designed to recirculate a portion of the engine's exhaust gases back into the intake manifold to lower combustion temperatures and reduce the formation of nitrogen oxides (NOx). When the EGR valve remains closed, exhaust gases are not recirculated, leading to higher combustion temperatures which directly contribute to an increase in NOx emissions.

The other potential observations related to exhaust temperature and fuel efficiency may also occur as a result of the EGR system's malfunctioning. A closed EGR valve could lead to higher exhaust temperatures since exhaust gases are not being mixed with fresh intake air to cool down the combustion process. Additionally, fuel efficiency might decrease because the engine may attempt to compensate for excessive NOx formation by altering fuel delivery or timing strategies.

However, the key aspect that unmistakably identifies the situation regarding a stuck closed EGR valve is the increase in NOx emissions, making it the most accurate observation in this case.