If a mechanic notices consistently negative fuel trim values across both banks, what could this suggest?

Consistently negative fuel trim values across both banks indicate that the engine control module (ECM) is trying to add fuel to the engine operation. This typically implies that the engine is running lean, meaning there is not enough fuel delivered relative to the amount of air entering the combustion chamber. The negative values suggest that the fuel mixture is below optimal levels, and as a corrective measure, the ECM is adjusting the fuel trim negatively to compensate for this imbalance.

In a lean condition, the combustion process can lead to higher combustion temperatures, which may affect the overall efficiency and performance of the engine. Components such as the fuel injectors, fuel pressure regulator, or intake leaks could be potential causes for the lean condition observed through negative fuel trim readings. Understanding this relationship between fuel trim values and engine performance is crucial for diagnosing issues that may arise during smog checks or general maintenance.