The STFT will start to climb immedi

The STFT will start to climb immediately to reflect the computer adding fuel. While the computer is adding fuel it still watches the o2 sensors until the o2 sensors are indicating that the lean condition no longer exists and proper air:fuel ratio is met. The computer will maintain this heightened addition of fuel until the leak is corrected. The scan tool will show STFT readings that are in the positive double digits, indicating that the computer has been adding too much fuel for normal operation. After a while the LTFT will also reflect this relative addition in fuel. Now if the vacuum leak is bad enough, the computer will not be capable of adding enough fuel to achieve proper air:fuel ratio. It will add fuel until the STFT reaches it's max calibration, usually 25%. Then a code is set in the computer indicating that the engine is running lean (P0171 or P0174) and the STFTs have maxed out. The opposite would be true if an engine was running rich due to a fuel leak (P0172, P0175).

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