What could cause high hydrocarbon (HC) emissions if STFT is around -5% and LTFT is at -18%?

High hydrocarbon (HC) emissions can occur when there is an improper air-fuel mixture in the engine. In this scenario, the short-term fuel trim (STFT) is -5%, and the long-term fuel trim (LTFT) is -18%. Both of these negative values indicate that the engine control module (ECM) is reducing the amount of fuel being injected into the engine relative to a baseline, suggesting that the ECM has detected an excess of air or insufficient fuel in the combustion process.

When a fuel injector is leaking, it can introduce additional fuel into the combustion chamber, leading to a rich mixture. Since the LTFT is significantly negative, indicating the ECM's attempt to compensate for an excessively lean condition, it suggests that there may be a miscalculation in the fuel delivery system. A leaking fuel injector can disrupt the intended fuel-air ratio, leading to incomplete combustion and higher hydrocarbon emissions as unburnt fuel escapes through the exhaust.

This understanding clarifies why selecting a leaking fuel injector as the cause of high HC emissions is reasonable, particularly in the context of the given STFT and LTFT values. Other options, such as a faulty mass air flow sensor or clogged fuel filter, could impact the air-fuel mixture