If the SFT value on a scan tool is negative, what can be inferred about the exhaust?

When the Short Term Fuel Trim (SFT) value is negative, it indicates that the engine control module (ECM) is adjusting the fuel mixture to run leaner. This adjustment generally implies that there is less fuel being added to the air-fuel mixture, which can correspond to conditions where there is an excess of oxygen in the exhaust.

In simpler terms, a negative SFT value means the system believes it has more fuel than required, and thus, it reduces the amount of fuel being injected to reach the optimal air-fuel ratio. Consequently, if the exhaust contains less fuel, it infers that there is a higher concentration of oxygen remaining in the exhaust gases, because less fuel means more air (and therefore oxygen) compared to the ideal combustion scenario.

This understanding reflects why the first choice, which states there is less oxygen in the exhaust, aligns with the conditions generated when the SFT is negative. Therefore, the context of SFT adjustments adds clarity to how fuel trims relate to exhaust composition and the oxygen levels present.