Change engine conditions, introduce a fault, and compare oxygen-sensor feedback with short- and long-term fuel correction.
Distinguish commanded fueling from closed-loop feedback; interpret the direction of short-term fuel trim (STFT) and long-term fuel trim (LTFT); and compare idle versus higher-load observations when investigating an unmetered-air fault.
Training model only: these values are illustrative, not OEM specifications or universal diagnostic thresholds. This lab uses a simplified wideband-equivalence-ratio display and does not reproduce a real sensor waveform or adaptive fuel-control algorithm.
Move time forward to see a conceptual shift from immediate correction toward learned correction. The same fault can show different trims at different loads.
Positive trim adds fuel; negative trim removes fuel. A near-zero trim alone does not establish that every component is healthy.
At warm idle, total fuel correction is strongly positive. At a higher load it becomes much closer to zero. Which fault pattern is consistent with that comparison?
Compare the same engine at idle and at higher airflow.
Fuel trim is evidence of how the control system is responding—not a diagnosis by itself. Confirm the complaint, check applicable codes and freeze-frame data, compare operating conditions and banks where available, and verify the suspected fault with vehicle-specific service information and appropriate testing.