# What is fuel trim on a car? ## Quick answer Fuel trim is the correction the engine computer makes to the amount of fuel it injects, based on oxygen sensor readings. ## What is Fuel trim? The engine computer starts with a calculated amount of fuel for the air coming in. Then it uses the oxygen sensors to see whether the engine is running rich or lean, and adjusts. That adjustment is called fuel trim, and it is shown as a percentage on a scan tool. The goal is to keep the mixture close to the ideal balance the catalytic converter needs to work. Positive numbers mean the computer is adding fuel because it sees a lean condition. Negative numbers mean it is taking fuel away because it sees a rich condition. Short term fuel trim changes quickly, and long term fuel trim is the learned correction over time. Adding the two together gives a rough idea of the total correction. Fuel trim is reported per bank. Bank 1 is the side of the engine with cylinder 1, and bank 2 is the other side on V6 and V8 engines. Comparing banks helps narrow down a fault: if only one bank is lean, a leak or fault on that side is likely, while both banks lean points to something they share, such as fuel pressure or the MAF sensor. When the correction gets too large, the computer sets codes such as P0171 or P0174 for lean, or P0172 or P0175 for rich. Vacuum leaks, a dirty MAF sensor and weak fuel delivery are common causes. ## How it works 1. The computer calculates a base amount of fuel from sensor readings such as airflow. 2. The upstream oxygen sensor reports whether the exhaust shows a rich or lean mixture. 3. Short term fuel trim adjusts fuel quickly, many times a second. 4. Long term fuel trim learns a steady correction, and codes set if it goes too far. ## Types of Fuel trim | Type | What it is | | --- | --- | | Short term fuel trim (STFT) | The fast, moment-to-moment correction that swings above and below zero as the oxygen sensor switches. | | Long term fuel trim (LTFT) | The learned correction stored over time, which shifts to keep short term trim near zero; large values point to a lasting problem. | | Bank 1 and bank 2 | Separate trims for each side of a V-type engine; inline four-cylinder engines usually show only bank 1. | ## Common Fuel trim problems - High positive trims at idle that improve at higher rpm, often from a vacuum leak. - High positive trims that get worse under load, often from weak fuel pressure or a clogged fuel filter. - Lean trims on both banks together, which can point to a dirty or failing MAF sensor. - High negative trims from a leaking injector, high fuel pressure or a faulty sensor reading. - Trims thrown off by an exhaust leak ahead of the upstream oxygen sensor, which lets outside air in. ## Frequently asked questions ### What is a normal fuel trim reading? Readings close to zero, often within about plus or minus 10 percent, are generally considered normal for long term fuel trim. Short term trim moves around constantly as the computer adjusts. Exact limits vary by vehicle, so compare with known good values for your engine when possible. ### What does high positive fuel trim mean? It means the computer is adding a lot of fuel because the engine is running lean. A vacuum leak, a dirty MAF sensor or low fuel pressure are common causes. If the long term trim gets too high, the computer sets a lean code such as P0171 or P0174. ### Can I see fuel trim myself? Yes, many scan tools and Bluetooth OBD-II readers show live fuel trim data. Looking at it at idle and at higher engine speed, around 2,500 rpm, can help narrow down the cause. Record the readings with the engine fully warmed up, since cold engines use different fuel strategies. ### What does negative fuel trim mean? Negative fuel trim means the computer is removing fuel because it sees a rich mixture. A leaking fuel injector, high fuel pressure, a stuck-open purge valve or a sensor reporting too much air can cause it. Large negative trims can set rich codes such as P0172 or P0175. ### What is the difference between short term and long term fuel trim? Short term fuel trim is the instant correction the computer makes many times a second. Long term fuel trim is a learned correction that builds up over time so short term trim can stay near zero. Long term trim shows lasting problems more clearly. ### Can a vacuum leak cause high fuel trim? Yes, it is one of the most common causes. Air that enters after the MAF sensor is not measured, so the engine runs lean and the computer adds fuel. Vacuum leaks usually show the biggest positive trims at idle, and the numbers often drop as engine speed rises. ### Does fuel trim reset when I clear codes? On many cars, clearing codes or disconnecting the battery resets long term fuel trim to zero. The computer then relearns it over the next drives. That is why a lean or rich problem can seem to go away briefly after codes are cleared, then return. ### What does bank 1 and bank 2 mean in fuel trim? Bank 1 is the side of the engine with cylinder number 1, and bank 2 is the opposite side on V6, V8 and flat engines. Four-cylinder inline engines usually have only bank 1. Comparing the two banks helps tell a one-sided problem from a shared one. ### Can bad gas affect fuel trim? It can. Fuel with water or a high ethanol content can change how the engine burns, and the computer may adjust trims to compensate. Flex-fuel vehicles are designed to handle different ethanol blends. If trims change right after a fill-up, fuel quality is worth considering. ### Can a bad oxygen sensor cause wrong fuel trims? Yes. The upstream oxygen sensor is what the computer uses to set short term fuel trim, so a slow or faulty sensor can make it add or remove fuel wrongly. Sensor codes often appear alongside, and a shop can check the sensor's response on live data. ## Sources - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Related terms - [Oxygen sensor](https://diagxcar.com/glossary/oxygen-sensor) - [Mass air flow sensor](https://diagxcar.com/glossary/mass-air-flow-sensor) - [Misfire](https://diagxcar.com/glossary/misfire) - [DTC](https://diagxcar.com/glossary/dtc) ## Related pages - [P0171 System Too Lean (Bank 1)](https://diagxcar.com/codes/p0171) - [P0174 System Too Lean (Bank 2)](https://diagxcar.com/codes/p0174) - [P0172 System Too Rich (Bank 1)](https://diagxcar.com/codes/p0172) - [P0175 System Too Rich (Bank 2)](https://diagxcar.com/codes/p0175) --- Source page: https://diagxcar.com/glossary/fuel-trim Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.