What could be the suspected issue if a vehicle has a smog test result of HC: 210 ppm and NOx: 150 ppm?

A smog test result indicating a high level of hydrocarbons (HC) at 210 ppm can suggest incomplete combustion of fuel, which often correlates with insufficient oxidation of exhaust gases. This can result from a clogged catalytic converter, which impairs its ability to convert harmful emissions into less toxic substances.

When the catalytic converter is clogged, it can lead to increased pressure back into the engine, affecting performance and combustion efficiency. The elevated levels of hydrocarbons could be a direct result of the converter being unable to adequately filter and burn off these emissions before they exit the exhaust system.

Meanwhile, the nitrogen oxides (NOx) reading of 150 ppm indicates a different reaction in the combustion process, often caused by high combustion temperatures, which may be influenced by other factors, but the high HC level still points to the catalytic converter issue as the primary concern in this scenario.

The other options suggest different problems: excessive spark advance typically leads to higher NOx emissions due to increased combustion temperatures, a rich fuel mixture generally causes higher HC emissions related to unburned fuel, and a faulty mass air flow sensor can result in incorrect air-fuel ratios leading to poor combustion, but the specific situation with the numbers provided strongly implies the catalytic converter is the