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On most obd2 cars, there are two measurements of fuel trim: short-term and long-term. Short term fuel trims(SFT) are the immediate readings that the ECU sees from the O2 sensor. These readings will fluctuate rapidly in a cycle pattern, usually around 0 if the O2 is working well.
Long term fuel trims I'm unsure about, but I think they are what the ECU sees as the direction that it is "bent" towards. Having a high positive number denotes that your car is running lean and will try to richen the mixture as you drive. Negative numbers is the opposite.
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