A car fails a smog test due to high HC (hydrocarbon) emissions. Which of the following could be the cause?

When a car fails a smog test due to high hydrocarbon (HC) emissions, one potential cause is a faulty spark plug. Spark plugs play a critical role in igniting the air-fuel mixture in the engine cylinders. If a spark plug is worn out, fouled, or malfunctioning, it can lead to incomplete combustion of the fuel. This incomplete combustion results in an elevated level of unburned hydrocarbons being emitted through the exhaust system, which contributes to higher HC emissions during the smog test.

In contrast, while other components such as the fuel injector, air filter, and oxygen sensor are important for proper engine performance and emissions control, their failure typically influences emissions in different ways. A malfunctioning fuel injector could result in a lean or rich fuel mixture, affecting overall efficiency but not always directly correlating with high HC emissions. An air filter that is clogged can lead to reduced airflow but is less likely to be the direct cause of high HC levels. Similarly, oxygen sensors primarily regulate air-fuel mixture ratios and monitor exhaust gases, which can lead to various emissions-related issues, but again, less specific to HC emissions compared to a faulty spark plug.

Therefore, a faulty spark plug is the most straightforward explanation for a significant