Long Term Fuel Trim
The ECU's permanent, learned correction to fueling. Unlike STFT which changes moment-to-moment, LTFT is the ECU saying "over time, I've learned I consistently need to add or remove this much fuel."
Why it matters
LTFT near 0% means the tune's fueling calibration matches reality. A large positive LTFT means the ECU is constantly adding fuel (tune is too lean). A large negative LTFT means it is constantly removing fuel (tune is too rich).
What to look for
Healthy is within +/-5%. Warning at +/-10%. Critical beyond +/-10%. Positive LTFT (lean correction) is always treated seriously.
Related terms
The ECU's real-time, moment-to-moment adjustment to fueling. Think of it as the ECU saying "right now I need to add a little more fuel" or "right now I need to pull back a little."
The ratio of air to fuel in the combustion chamber. Lower number = richer (more fuel). Higher number = leaner (less fuel).
Another way to measure air-fuel ratio, expressed as a ratio where 1.0 = perfect stoichiometric combustion. Below 1.0 = rich. Above 1.0 = lean.
Want to learn more about Long Term Fuel Trim?
Get our free fuel system logging guide -- covers the channels you need, what to look for, and how to interpret your data like a pro.
See Long Term Fuel Trim in action
Upload your datalog and every finding, including LTFT, will be explained in plain English with actionable next steps.
Analyze your own log