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The fuel trim values determine the amount of fuel the engine requires for a complete combustion to occur (direct reactions to upstream oxygen sensor’ response). 
This sequence of information gathering is controlled by the ECM, the value depends on feedback from other components (sensors and actuators) between the air intake duct to exhaust mufflers but mainly the Mass Air Flow meter (MAF), Intake Air Temperature sensor (IAT), Manifold Absolute Pressure sensor (MAP) and Oxygen sensor are the initiators off the sequence.
About to read the Fuel Trim values using an Autoboss Diagnostic Scan tool.
For proper understanding of the discussion, the need to understand oxygen sensor function with respect to fuel trims will be inevitable, we need to have functional oxygen sensors in the vehicle before we can effectively treat fuel trim, so for now we are assuming we have functional oxygen sensors, we will take a detailed look into it in future case studies.

Complete combustion occurs when the Stoichiometric value of 14.7 part of air is mixed with 1 part of fuel all in grams per second (g/s), without any harmful residual gas after combustion.
This implies that incomplete combustion is caused by poor stoichiometric values which in turn affects the environment and the operation of the automobile. 
The fuel trim values are used to compensate for other drivability issues within the ECM designed threshold, it also provides an overall picture of the issues from intake manifold leak (+% fuel trim) to a stuck open injector (-% fuel trim).

The positive and negative fuel trim values respectively determines the extent to which the vehicle is running LEAN or RICH.
When the vehicle is running LEAN the positive percentage (+%) implies ADD fuel likewise when it runs RICH the negative percentage (-%) implies SUBTRACT fuel.
 
To properly diagnose engine operation and drivability issues in automobiles a detailed understanding of fuel trim value and related PIDs is required. 

The fuel trim value is classified into two (2) terms, Long Term Fuel Trim (LTFT) and Short Term Fuel Trim (STFT) and are measured in percentage (%) of fuel added to or subtracted from. For standard operation the sum of the STFT and LTFT should not be more than + or – 15% or else a Lean or Rich situation will be registered by the ECM.
Fuel Trim Figures
SHORT TERM FUEL TRIM (STFT)
This is the immediate response by the ECM to adjust the Air/Fuel ratio by adding or subtracting fuel to correct either a LEAN or RICH condition respectively, in quick reaction to oxygen sensor input values. During closed loop operations the ECM uses the STFT calculations to constantly command the air/fuel ratio slightly lean and then slightly Rich. The STFT is not stored in the keep alive memory (KAM), it automatically resets to ZERO (0) once the vehicle is restarted.

A good example of this response can be practically tested by causing a large leak while the vehicle is idling on any of the vacuum hoses connected to the air intake manifold, the ECM will react immediately by adding more fuel to balance the air/fuel mixture. 

The STFT’s main functions are to keep the oxygen sensor near Stoichiometric levels, command the ECM to alter injection pulse width and 0% fuel correction from factory preset value, all these occurs only during close loop.
LONG TERM FUEL TRIM (LTFT)
This value is learned over time while the vehicle is in close loop operation and is stored in the KAM unlike the STFT, as the engine wear out due to long usage, the cylinders looses its sealing capabilities causing the STFT to trend high or low most of the time, the ECM has the ability to learn trends and store them in its memory. 
These stored fuel trim values will be used to calculate and compensate for the changes causing the trends, since the fuel trim is an ongoing calculation, keeping one factor in the calculation constant makes it possible to return STFT to its normal range, enabling a faster and more accurate response to changes in operational conditions as acceleration and deceleration. These changes in operational conditions take between 5 to 20 seconds for the LTFT to respond since it’s stored in the KAM. 
The LTFT functions to keep the STFT within manufacturer’s operational specification. Its value are also used during open loop fuel calculations (during start up and WOT), its adjustment are very coarser.
The LTFT’s main functions are to keep the STFT as close to 0% as possible and retain any fuel correction memory. The PCM looks at the LTFT first to know where to start injector pulse width, it learns from the STFT, + or – 10% is considered normal on most system and may be used during open and close loops.

On most scanners the fuel trim value can be observed in MODE 1 interface under the Live data or current data depending in the type or level of scanner you are using. As stated above the values are between + or – 1 to 15% any value more or less will be seen as a malfunction by the ECM.
HOW TO CHECK FUEL TRIM USING A SCAN TOOL.
This function is not dependent on the type or brand of scan tool you are using, the same procedure is used, the only difference is on the interfaces applied by the different scan tools. 

Below are pictures to assist you.
In this case we are using the Autoboss V30.

For further clarity feel free to ask questions.

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