Which issue could cause a car to run lean and fail a smog check due to high NOx emissions?

Running lean means that there is an insufficient amount of fuel in the air-fuel mixture entering the engine. This condition can lead to higher combustion temperatures and increased levels of nitrogen oxides (NOx) emissions, which can cause a car to fail a smog check. A faulty mass airflow sensor is crucial because it measures the amount of air entering the engine. If it malfunctions, it may provide incorrect readings to the engine control unit (ECU), which can lead to an improper fuel injection strategy. As a result, the engine may receive too little fuel, causing a lean mixture and subsequently higher NOx emissions.

In contrast, a defective catalytic converter would primarily affect the vehicle's ability to reduce harmful emissions after combustion and would not directly cause a lean running condition. Worn spark plugs can lead to misfiring and inefficient combustion but are not typically the direct cause of a lean condition. Oil contamination can result in other issues such as excessive smoke or oil burning rather than a lean fuel mixture. Thus, the mass airflow sensor is critical for proper air-fuel ratio management, making its malfunction a likely cause of high NOx emissions due to the engine running lean.