Fuel Trim Interpreter (STFT / LTFT)
Enter your short- and long-term fuel trim values and get a plain-English diagnosis: lean vs rich, vacuum leak vs fuel delivery, and what to check. Free OBD2 tool.
Read short-term (STFT) and long-term (LTFT) fuel trim off your scan tool, in percent. Idle is required; add a cruise reading (~2500 rpm, steady) to separate a vacuum leak from a fuel-delivery problem.
What fuel trims tell you
Fuel trim is the engine computer's confession. Short-term (STFT) is its live correction based on the oxygen sensor; long-term (LTFT) is the learned average it carries over time. Add them together: near zero means the engine is running as designed. Sustained positive means it's lean (adding fuel to compensate for extra air or missing fuel); sustained negative means it's rich (pulling fuel out).
The idle-vs-cruise trick
One reading rarely tells the whole story — the pattern does. Lean at idle but normal at cruise is the fingerprint of a vacuum leak: the unmetered air is a big fraction of the small idle airflow but a trivial fraction at speed. Lean everywhere points at fuel delivery or the MAF instead. That's why this tool asks for both. Fuel trims are also the key to codes like P0171 (system too lean).