// Diagnostics

Reading Fuel Trims (STFT and LTFT): A Practical Guide

Published by DiagEdge · 17 October 2026 · 6 min read

Fuel trims are the most useful live-data values on a petrol engine, and the ones most techs underuse. Learn to read them and a huge range of driveability and emissions faults — lean codes, rich codes, misfires, P0420 — start pointing at their own causes.

What the trims are

The ECU targets a stoichiometric air-fuel ratio and adjusts injector time to hit it, guided by the oxygen sensors. Those adjustments are reported as:

• Short-Term Fuel Trim (STFT) — the immediate, moment-to-moment correction. It bounces around constantly.
• Long-Term Fuel Trim (LTFT) — a learned, slower average that the ECU stores so it doesn't have to make the same big short-term correction every time.

The sign is everything:

• Positive trim = the ECU is ADDING fuel → it's correcting for a lean condition.
• Negative trim = the ECU is REMOVING fuel → it's correcting for a rich condition.

What's normal

Roughly, ±5% is healthy, up to about ±10% is usually acceptable, and beyond ±10% is worth investigating. Many ECUs set a lean/rich code around ±20–25% when they run out of correction. Read STFT and LTFT together — a big LTFT with STFT hovering near zero means the engine has learned a correction for an ongoing fault.

Read them at idle AND at load

This is the trick that turns trims from a number into a diagnosis. Note the trims at idle and again at a steady cruise / higher load, and compare:

• Lean (positive) worse at idle, better under load → vacuum leak. The fixed leak is a big share of the small idle airflow, and shrinks as airflow rises.
• Lean worse under load, fine at idle → fuel delivery can't keep up with demand (pump, filter, pressure).
• A roughly constant offset at all loads → often a MAF reading consistently high or low.
• Rich (negative) trims → too much fuel: leaking injector, high fuel pressure, or a MAF/sensor reading high.

Worked examples

• LTFT +18% at idle, +6% at cruise: classic vacuum leak — smoke-test the intake.
• LTFT +4% at idle, +16% at cruise: fuel delivery under load — test fuel pressure while driving/loading.
• LTFT −14% across the board: rich everywhere — suspect fuel pressure high or a leaking injector; check MAF.

Why it ties everything together

A lean code (P0171/P0174), a random misfire (P0300) and even a catalyst code (P0420) can all trace back to the same underlying mixture fault — and the trims are how you see it directly. Before replacing sensors or cats, read the trims at idle and load. More often than not, they name the cause.