What Is a Fuel Trim Cell?

A fuel trim cell is a tiny computer in a car that helps to regulate the air/fuel mixture delivered to the engine. It’s integrated with the car’s powertrain control module PCM which takes information from different sensors and makes adjustments to the fuel injection system to ensure the engine is running properly. 

So you just read the scanner data and got to know about your engine performance. One such data is related to fuel trimming in your engine. 

But what is a fuel trim cell? 

Well, that clears up what a fuel trim cell is. But how many types are there? Or should you even read the fuel trim numbers? Read this article to find out. 

What Does Fuel Trim Mean?

Fuel Trim is the process of fine-tuning the air-fuel ratio in the engine. It is to make sure that the engine is running efficiently.

Correct air-fuel mixture ensures that the engine has proper fuel economy. Fuel trimming is the process of keeping the air-fuel ratio in check. 

It does that through the car’s fuel management system which includes the fuel trim cell. Based on the data received from the different sensors, fuel trimming takes place and the appropriate air-fuel ratio is maintained. 


Fuel trim is the process of controlling the air-fuel ratio in the engine based on the data from the sensors.

How Does Fuel Trim Cell Work?

Fuel trim cell gets the information from different sensors like mass air flow MAF sensor, O2 sensor. These two sensors send the data of both intake and exhaust to the fuel trim cell.

Based on the data, this trim cell signals the fuel injector to increase or decrease the fuel flow to the engine. Thus maintaining the proper air-fuel ratio for prolonged engine operation. 

Types of Fuel Trim

There are two categories based on the duration for which the fuel trim happens. These are Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT).

What is a Short Term Fuel Trim (STFT)?

Short term fuel trim is when the adjustment happens immediately. For example if the engine is running lean, then the fuel trim cell instructs the fuel injector to add more fuel to the system.

Let this table clarify the short term fuel trim process. 

Signal from the O2 SensorEngine Fuel ConditionImmediate Action by the Fuel Trim CellFuel Trim Number
Voltage = 0.45Perfect AFRN/A0
Voltage Above 0.45RichInstructing fuel injectors to reduce fuel flowNegative (-)
Voltage Below 0.45LeanInstructing fuel injectors to increase fuel flowPositive (+)

We have to understand the O2 sensor. O2 sensor keeps track of the exhaust mixture and signals this data to the fuel trim cell. 

The O2 sensor sends a signal between 0.1 to 0.9 V. If it is a low signal, such as 0.1 V then it means that the engine is running lean and more fuel is needed. Based on this data, the fuel trim cell signals the fuel injector to send more fuel to the combustion chamber.

Similarly, if the O2 sensor sends high voltage signals such as 0.89V then it means that the engine is running rich and less fuel is needed. Based on this data, the fuel trim cell signals the fuel injector to reduce fuel flow to the combustion chamber. 

So, this is how short term fuel trim works to maintain the appropriate air-fuel ratio.


O2 sensor sends low or high voltage signal to the fuel trim cell. Low voltage means more fuel is to be added and vice versa. This quick short term adjustment means short term fuel trim. 

What is a Long Term Fuel Trim (LTFT)?

Long Term Fuel Trim helps to trim fuel for a problem that takes place for a long time period. It monitors short term fuel trim (STFT) numbers and takes action. 

Let’s see an example.

Suppose, there is a clogged fuel injector. It means less fuel flow and the engine runs in lean condition. 

Short Term Fuel Trim action will be to instruct the fuel injector to add more fuel. But since the fuel injector is clogged, it will keep injecting less fuel than required.

So, the problem remains and that’s when the Long Term Fuel Trim gets involved. It starts trimming while the short term fuel trim number drops to 0.

In the meantime, short term fuel trim will solve smaller and more immediate problems. 

Now, we want to clarify here that even though LTFT will help with this process, you still need to diagnose and fix your fuel injectors. Fuel Trim cell help you with the fuel input process and not help you fix the components itself. 


STFT is fuel trimming on a regular basis to adjust the correct air-fuel ratio. But LTFT is keeping track of STFT’s performance and averaging it around 0. 

What are Normal Fuel Trim Values?

A total normal fuel trim range should be between + or – 10% in total.

For total fuel trim being more than 10%, your air-fuel mixture is lean. That means there is either a problem with 

  1. Vacuum leaks from intake gaskets
  2. Unclean mass airflow sensor
  3. Clogged Fuel Injector
  4. Clogged Catalytic Converter

Similarly, for total fuel trim being more than negative 10%, the air-fuel mixture is rich. So, there is again a problem with

  1. Dirty Mass Airflow Sensor
  2. Leaking Fuel Injector
  3. Fuel Pressure Regulator

And of course, your O2 sensor could be damaged as well. The point is that it helps you to identify potential problems in the system and make necessary fixes. 

You get a fault code when that happens. Some of these fault codes are

  1. P0170 – Fuel Trim Bank 1
  2. P0171 – System Too Lean (Bank 1)
  3. P0172 – System Too Rich (Bank 1)
  4. P0173 – Fuel Trim Bank 2
  5. P0174 – System Too Lean (Bank 2)
  6. P0175 – System Too Rich (Bank 2)

Whenever you find that the fuel trim number exceeds 10%, you can work to solve problems related to your engine. 

Now, you need to know how to get this fuel trim data. 

How Do You Get Fuel Trim Data?

You need to scan for fuel trim on three different engine conditions. These are idle, at 1500 rpm and at 2500 rpm. After doing that, check the fuel trim numbers. Follow these steps to read the fuel trim data

Step 01: Grab the OBD II Diagnostic Scanner and attach it to the connector in your car. It must be near the steering column of your car.

Step 02: Turn on your engine and start the scanner. It will start communicating with the car’s computing system.

Step 03: Insert all the information related to your car’s model and engine VIN code.

Step 04: Your scan tool will read the data and display a set of outputs.

Some key readings from this scan tool are

  • STFT B1 – Short Term Fuel Trim Engine Cylinder Bank 1
  • STFT B2 –  Short Term Fuel Trim Engine Cylinder Bank 2
  • LTFT B1 –  Long Term Fuel Trim Engine Cylinder Bank 1
  • LTFT B2 –  Long Term Fuel Trim Engine Cylinder Bank 2

Now, what does Bank 1 and 2 mean? Bank 1 means the engine that comes first in the firing order. Bank 2 is at the opposite side of Bank 1. Please check the firing order for your car model.

For the STFT and LTFT, you will find the engine firing number adjacent to it. 

Note that, the fault codes mentioned previously will only show up if there is any problem in the system. Otherwise, you will only get data related to the STFT and LTFT. 

Sometimes, there will be cases where before starting your car the fuel trim number will be much higher. It means that your car is in open-loop condition.


The normal range is within 10%. Reading the fuel trim number is easy. All you have to do is connect the scanner to the connector and read the data. Of course, there are some fault codes, and the meaning of each has been added.

Finally, by this point, this article should clear out most information related to Fuel Trim Cells. Its types, functionalities, and some of the problems. 


What is Fuel Trim Percentage?

It means the adjustment your car’s Electronic Control Module (ECM) does to maintain the proper air-fuel ratio. Based on the requirement, it will either increase (positive fuel trim) or decrease (negative fuel trim) the fuel quantity. Every engine has a specific range. Check your car’s user manual to find out what’s yours. 

Does Fuel Trim Cell Affect Engine Fuel Economy?

Of course it does. The main purpose of a fuel trim cell is to maintain an appropriate air-fuel ratio. Failing to do so means either a lean or a rich mixture. Neither of those are good for the fuel economy and the overall engine performance. 

Does Damaged PCM Affect the Fuel Trim Performance As Well?

Yes, it does. PCM is how you get the fuel trim performance. It helps to monitor the performance of different sensors such as O2 sensors. PCM then uses this data to make necessary adjustments on the fuel injectors and maintain the air-fuel ratio. Damaged PCM means inaccurate fuel trim performance. 


So now you know what a fuel trim cell is and its types. Hopefully, our guide has made it clear about its uses. Meaning when you should use long term or short term fuel trim. With that out of the way, make sure to use the best quality fuel trim cell for your car.

Leave a Comment

Written By

Photo of author

William Baldwin

This Is William Baldwin, The Founder & Managing Editor of this website. Me and my team share automotive tips, tricks, and news

Fact Checked And Mechanically Reviewed By


Talha Atta

Talha Atta, a Mechanical Engineer and experienced technical content writer and editor at Autoglobes.com with a passion for the automotive industry.