If a car is running rich and shows STFT at -12% and LTFT at -15%, what is the potential issue?

When a vehicle is running rich, it means that the engine is consuming too much fuel relative to the amount of air, resulting in an overly rich fuel mixture. The short-term fuel trim (STFT) and long-term fuel trim (LTFT) values provide insight into how the engine management system is adjusting fuel delivery in response to sensor data. In this scenario, negative STFT and LTFT values indicate that the engine control unit (ECU) is reducing fuel delivery to compensate for an excess of fuel.

A malfunctioning mass air flow (MAF) sensor is likely to cause this rich condition because it measures the amount of air entering the engine. If the MAF sensor is providing incorrect readings, such as underreporting the air flow, the ECU may incorrectly perceive that there is less air than there actually is. As a result, the computer may continue injecting too much fuel, leading to a rich condition and the corresponding negative fuel trims as the system attempts to correct the mixture.

The other potential issues listed do not directly explain the negative fuel trim values as clearly. A faulty oxygen sensor could lead to incorrect feedback, but it typically would manifest differently. A clogged fuel filter could affect fuel supply but would not specifically cause the negative fuel trims