What does short-term fuel trim 99.2% mean?

[DelLincoln]

Short-term fuel trim refers to the engine control unit (ECU) adjusting the fuel injection pulse width based on the oxygen sensor readings. If no issues occur, the malfunction indicator light will illuminate when the air-fuel mixture is lean, and the system will adjust it towards a richer mixture. Below are detailed explanations: 1. System Control: During open-loop control, the injector has a fixed base injection pulse width corresponding to the air mass measured by the mass airflow sensor or the load measured by the manifold absolute pressure sensor. This pulse width is adjusted based on factors such as engine temperature and engine speed. During closed-loop control, the pulse width may either increase or decrease, allowing positive or negative adjustments to ensure an appropriate air-fuel mixture under various operating conditions. 2. Air-Fuel Mixture: When the mixture is too rich, the oxygen sensor output voltage increases, and the short-term fuel trim decreases, meaning the fuel injection pulse width will shorten, resulting in a value less than 1 on the diagnostic tool. Conversely, when the mixture is too lean, the short-term fuel trim increases, and the value displayed on the diagnostic tool will be greater than 1.