What LTFT and STFT mean when positive or negative
LLCAR Editorial Team · published
STFT is the current correction to fuel delivery. LTFT is the long-term correction that the control unit has accumulated for a given operating mode. A positive value means the control unit is increasing delivery relative to the base calculation. A negative value means it is reducing it. Look at both values at the same time and separately for each bank of cylinders.
What the two trims show
| Reading | How to interpret it |
|---|---|
| STFT and LTFT positive | Both trims are directed toward increasing delivery. Note whether this repeats on a warmed-up engine and in which operating mode. |
| STFT and LTFT negative | Both are directed toward reducing delivery. Record at what rpm and load this happens. |
| One positive, the other negative | The trims partially compensate for each other. Save both, even if their sum is close to zero. |
Identical trims can occur with different faults. Before replacing a sensor, injector, or another part, match the codes to the conditions in which the deviation repeats, and confirm the suspected cause with a separate check.
Why the numbers differ at idle and while driving
The engine runs at different rpm and load, so the control unit may use different accumulated corrections. First compare idle with idle, and a steady stretch of a trip with a similar stretch of another trip. Then look at which operating mode the deviation repeats in. A consistent positive only at idle and a consistent positive while driving give a mechanic different conditions for finding the cause.
Compare readings at a similar warmed-up engine temperature and in closed loop. In this mode the control unit corrects the mixture based on feedback from the exhaust sensors. If the scanner shows STFT Bank 1 and LTFT Bank 1, compare them with each other. For Bank 2, use a separate pair if the vehicle transmits it. Record a zero reading and missing data separately.
What to save for diagnostics
- Record the STFT and LTFT of every available bank of cylinders at warm idle. Add temperature, rpm, load, and loop status if this data is available.
- If you have a recording of a regular trip, select a stretch of steady driving after stopping. Save the same parameters and note where the deviation persists and where it appears only momentarily.
- Save the exact fault code and the conditions of its occurrence before clearing it. If the deviation repeats, give both measurements and the codes to the mechanic. Acceptable values and the checking procedure depend on the engine.
The Android build of LLCAR AI0.19 provides analysis of STFT and LTFT for each bank of cylinders. Comparing idle with operation under load requires the corresponding data from the vehicle. How to read engine codes with your phone · What to check with Check Engine · P0171 and P0174: 5 diagnostic steps.