Adaptive Learning / Fuel Trim
Fuel trim is a two part adaptive strategy that adjusts fuel injector pulse width (open time) to adapt to wear, defects, or changes in the system. The two parts of a computers fuel trim adaptive strategy are short term fuel trim STFT (additive) and long term fuel trim LTFT (multiplicative).
The vehicle's operator controls air intake into the engine's cylinders through the accelerator pedal and the PCM controls the amount of fuel according to these and other demands. It tries to maintain a long term air/fuel ratio of 14.7:1. If it sees that the long term ratio is not maintained at an acceptable range for STFT, it adds or subtracts fuel to the mixture to compensate. Injector on time is the result of sensor input and learned adaptive strategies implemented by the PCM. When LTFT exceeds 10%, the PCM is likely compensating for a faulty component, something beyond normal engine wear.
The illustration above shows the different sensors used to calculate fuel injector pulse width. The question indicates the importance of oxygen sensor input in calculating injector pulse width. The PCM's fuel trim is based on feedback from the oxygen sensor. The PCM makes minor adjustments to the fuel ratio and checks the results with the O2 sensors input. If the oxygen sensor were stuck frozen with an erroneous and constant output of 800Mv, the injector pulse width and air/fuel ratio would be affected.