Even when it’s not working properly, your car’s engine is constantly making changes to the way it both takes in, and uses, the air and fuel it needs for combustion. Everything from your lead foot, to the annoying rain storm you just drove through, can cause the engine to need a different air-fuel mixture to run properly.

Through processes that are known as short term fuel trim and long term fuel trim, your car’s computer adjusts the amount of fuel that flows into the engine to compensate for changes in external and internal conditions. The good news is that you don’t need to know how to make those changes yourself, but the bad news is that, in the event of an issue, you’ll need to be at least marginally aware of what’s happening in your engine.

Car Bibles’ editors have spent enough time banging their heads against the wall to figure out how fuel trim works, and they’re eager to share their hard-earned knowledge with you here.

Let’s get started.

What Is Fuel Trim?

Your car’s engine control unit, or ECU, uses fuel trim to adjust and compensate for changes in the air/fuel mixture. Since engines rely on a precise ratio between the two, any deviation from that mix can cause problems.

How Does Fuel Trim Work?

Fuel trim helps engines compensate for changes in the air/fuel mixture, but more importantly, it helps engines deal with external factors that can cause unexpected changes, such as weather, moisture, temperature, and other internal issues like sensor failures.

What Is The Difference Between Short Term And Long Term Fuel Trim?

Just as their names suggest, long term fuel trim and short term fuel trim deal with different issues inside the engine. Long term fuel trim refers to the process through which the quantity of fuel is adjusted in response to changes over long periods of time. This can have to do with issues like fuel injector problems and other changes. These changes are stored permanently in the car’s memory.

Short term fuel trim refers to the changes made to respond to current conditions. The updates happen rapidly—sometimes as often as several times per second—and are not permanently stored in the vehicle’s memory banks.

Car Bible’s Glossary for Fuel Trim

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Air-Fuel Mixture

Your car’s engine relies on a specific ratio of fuel and air for proper combustion and proper timing of that combustion. Too much or too little, and the engine can backfire or struggle to produce power.


The ECU, or engine control unit, has several jobs. It controls things like the air-fuel mixtures and fuel trim, and also monitors the performance of various sensors and systems. That annoying check engine light you see in your car’s dash is the result of a decision the ECU made.

Fuel Injector

The fuel injector is the mechanism that sprays fuel into the cylinder. The flow and amount of fuel delivered is controlled, in part, by fuel trim and the ECU.

The Car Bibles Questionnaire on Fuel Trim

Car Bibles answers all your burning questions!

Q. Can I Reset Long-Term Fuel Trim?

A. Yes, you can, and it’s sometimes necessary. The process will vary, depending on your car’s make and model, but there’s generally some combination of warming the vehicle up, disconnecting the battery or removing the ECU/ECM fuse, turning the ignition on, and waiting for the system to cycle through its checks.

Q. How Will I Know If My Fuel Trim Is Off?

A. You might not know that the fuel trim is off, specifically. You’ll notice other issues like poor fuel economy, loss of power, or a check engine light. Sometimes, fixing the issue is as simple as repairing or cleaning the fuel injectors, but you’ll need a mechanic or know your way around an OBD-II scanner to figure it out.

Q. How Can I See My Fuel Trim Numbers?

A. Many OBD2 scanners will show you both short and long term fuel trim. In most engines, a healthy short term reading is between +/- 10 percent and a long term reading between +/- 5 percent.

Video on Fuel Trim

For all you kinesthetic learners out there, Car Bibles brought you a video from one of our favorite and most trusted, sources. You’re welcome.

Car Bible’s Favorite Fuel Trim Related Products

Understanding fuel trim can be complicated and annoying. That’s why Car Bibles’ editors have picked a few of our favorite products to get you started with diagnosing and reading fuel trim and related issues. They include the Innova Code Scanner, Mechanix Work Gloves, and Pro-Lift Jack Stands.

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