Fuel Trim Chart Positive or Negative Numbers

According to the chart 128 is neutral. I have a snapon MT2500. When my o2 is unplugged the ST and LT Fuel Trims stay on 128. When o2 is plugged in my STFT bounces between 0 and 40 give or take some. But the numbers do not show a plus or a minus next to the numbers. My LTFT has gotten down to 62. Again no plus or minus sign by the number.

QUESTION:
Because my numbers are below 128 on a scale from 0 to 255 does that mean my Fuel Trim numbers are considered Negative or Positive according to the chart?

(Note) As described above when my STFT hits 0 and bounces up to 40, where in the chart is 0?

Below 128 is subtracting fuel, above 128 is adding fuel.
62 LTFT would indicate the 02 sensor is leaning out meaning you have a rich condition. and the computer compensated for this be subtracting fuel.

Time to find the cause of the rich condition.

The only factor I didn't mention was that I made some repairs and the LTFT has been decreasing, I think due to ECU relearning.

I don't suppose that because the LTFT has been decreasing that it makes any difference regarding a rich condition, as far as it correcting itself. Would that be incorrect to make that assumption?

Thanks again John.

After a repair your short term fuel trim should be on the opposite side of the long term to get the value as close to 0 or 128 as possible 118 to 138 is a good range.


Example
Before repair of a lean condition it might look like
LTFT +17
STFT + 3
Total fuel trim = +20

After repair it would be something like this
LTFT +17
STFT -15
Total fuel trim +2 (+/- 10) on a good fix

randy61 wrote: The only factor I didn't mention was that I made some repairs and the LTFT has been decreasing, I think due to ECU relearning.

I don't suppose that because the LTFT has been decreasing that it makes any difference regarding a rich condition, as far as it correcting itself. Would that be incorrect to make that assumption?

Thanks again John.

The LTFT will take some time to get back into range. But total fuel trim is the key indicator and confirmation of a fix