What is a potential cause of poor fuel economy indicated by STFT of -8% and LTFT of -17%?

The indication of short-term fuel trim (STFT) at -8% and long-term fuel trim (LTFT) at -17% points to an issue related to fuel delivery. The negative values suggest that the engine control unit (ECU) is adjusting the fuel delivery to reduce the amount of fuel entering the combustion chamber, which frequently occurs due to excess fuel being delivered.

A leaking fuel injector can cause this condition, as it allows fuel to enter the combustion chamber when it shouldn't be, leading to a rich air-fuel mixture. In response, the ECU compensates by reducing fuel to maintain the appropriate air-fuel ratio, which explains the negative fuel trim readings. This consistent adjustment reflects the engine's ongoing need to counteract the effects of the leakage, ultimately contributing to poor fuel economy.

Other options such as a bad spark plug, a worn air filter, or a faulty ECU could cause performance issues or changes in fuel economy as well, but they would not specifically result in the same characteristic fuel trim readings observed here. A bad spark plug could lead to misfires or incomplete combustion, potentially affecting fuel efficiency but wouldn't directly create the fuel trim scenario described. Similarly, a worn air filter might restrict airflow and affect performance, but would not