Fuel trim explained (STFT and LTFT)

Fuel trim is how far the ECU is correcting the mixture away from its base map. STFT (short term) moves quickly. LTFT (long term) is the learned offset.

How to read the numbers

• Around 0% is healthy.
• Positive trim (for example +15%) means the ECU is adding fuel — the engine looks lean.
• Negative trim (for example −12%) means the ECU is pulling fuel — the engine looks rich.

Codes that live here

P0171 / P0174 are lean limit codes. P0172 / P0175 are rich limit codes. High trims with a misfire often point to vacuum leaks or MAF issues, not “just replace O2 sensors.”

Bank 1 vs bank 2

On V engines, compare banks. If both banks are equally lean, suspect MAF or a leak after the MAF. If only one bank is lean, look at that bank’s intake, injectors, or exhaust leak at the manifold.

MotorBit includes a fuel trim screen so you can see STFT/LTFT without a laptop.