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Reading Fuel Trim in Live Data

Published on January 3rd, 2024

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Fuel trim is critical when it comes to diagnosing lean and rich conditions. It determines how the ECU controls the engine's fuel delivery. Always verify the STFT (short-term fuel trim) and the LTFT (long-term fuel trim) while reading live data from your Innova OBDII scan tool.

The scan tool displays both the STFT and the LTFT as a percentage. The optimal reading for the STFT and LTFT is between 5% and 10%. Be worried of anything above 10% or below -10%.

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High fuel trim percentages indicate that the engine computer is trying to compensate for lean situations in the motor. When your car has this issue, which usually comes with the code P0171 and/or P0174, the first thing to do is check the gas cap. It may be loose or faulty, and all you need to do is tighten or replace it. If the gas cap isn't the problem, there are 2 possible scenarios:

  • Vacuum leak - usually caused by a crack in vacuum hoses
  • Faulty sensors – there are different types of sensors that can be faulty, usually O2 sensors, the fault codes should show specifically which one(s) is not in the right shape.  

A negative fuel trim, on the other hand, indicates that the CPU is attempting to alter the fuel mixture to compensate for a rich mixture. This rich condition will come with the codes P0172 and/or P0175.

One of the most common symptoms of the rich condition is that your car runs fine when it’s cold but starts to lose power when the engine warms up.

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Using Live data to look at fuel trims and O2 sensors in a 2018 Ford Transit.

When your car has this problem, it can be because the sensors are blown (MAP, IAT, O2, ECT, fuel rail pressure sensor, etc.). These faults can be easily navigated by reading the specific codes.

In some vehicles, those sensors may use the vacuum mechanism. The sensors may not be faulty, but there could be vacuum leaks in the hoses attached to them.

If you can sense a heavy gasoline smell, leaking fuel injectors are very likely to be the cause for the rich condition.

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