Thanks for the detailed reply Darrell. I am still struggling to see the error in the first statement that "STFT is used to keep the O2 sensor moving slightly rich/lean from stoich" When I watch the video, this is my interpretation:

The desire of the ECU is to see the O2 sensor voltage cross the .450V line, repeatdely. In order to see this, it continually adjusts the fuel pulse until it cannot do so anymore. The fuel pulse is determined by the total trim. I think of the LTFT as a constant (at least in the short term :)) , and the STFT as the final "tweak" factor to get the voltages crossing the .450 line. I never got the impression from his video that the O2 sensor knew how much richer or leaner the mix was from stoich.

Is anything in my interpretation inaccurate?

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Scott Amoroso
Technician
Scott Amoroso
Virginia Beach, Virginia, USA