What do short-term and long-term fuel trim numbers mean?
Short answer: Fuel trims show how much the computer is adding or removing fuel from its base calculation; positive numbers mean it is adding fuel because it sees lean, negative means it is pulling fuel because it sees rich.
Short-term fuel trim reacts quickly to oxygen sensor feedback and swings constantly. Long-term fuel trim is the learned correction the computer keeps when short-term trim stays off center. Add them together to see the total correction on that bank.
As a rough shop guide, total trim within about plus or minus 10 percent is usually normal. Totals beyond plus or minus 20 to 25 percent typically set a lean or rich code, though the exact threshold varies by manufacturer.
The diagnostic power comes from comparing conditions. High positive trim at idle that drops toward normal at 2500 rpm points to an unmetered air leak, because the leak matters less as airflow rises. High positive trim that gets worse under load points toward fuel delivery, such as a weak pump or restricted filter; confirm with a fuel pressure gauge, and wear eye protection with a rag around the fitting when you connect it. Compare banks too: one bank lean suggests a bank-specific leak or injector, both banks lean suggests something common like airflow measurement or fuel pressure.
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General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.