I have a 2002 Toyota Corolla (1ZZ-FE engine) and I am trying to understand the fuel trim values I am getting in real driving conditions from my scan tool. I have P0171 code (lean condition) that comes on and off. I posted earlier about this, but I fixed a lot of things in the meantime. I was hoping someone could take a look at the screenshot attached and tell me if these values look off or not.

  • STFT mostly remain between -5% - 5% which I think is great.
  • LTFT, however, ranges from 12 to 25% These values look high to me, is that right?

I appreciate any insights!

ST and LT fuel trims

  • 1
    Yes, 25% is too high.
    – vini_i
    Commented Dec 15, 2016 at 22:53
  • 4
    Fuel trim high at low rpm and better at higher engine speed. The first thing I look for is a vacuum leak. Commented Dec 16, 2016 at 4:56

1 Answer 1


@Fred Wilson's comment is right on the money, high, positive long term fuel trims at idle that reduce at higher engine speeds indicate the presence of higher-than-normal unmetered air.

The fact that it's unmetered indicates that you need to look for something after the MAF/MAP sensor.

Things to check include:

  • Unplugged/damaged hoses (e.g. EGR, PCV)
  • Cracks in the intake manifold
  • Leaky intake manifold gaskets
  • Leaky brake booster vacuum line

Fuel management picks up the presence of excess air at exhaust via the O2/lambda sensors and adjusts for it by keeping the fuel injectors open for longer.

This question lists some possible methods to discover the leak source(s):

How can I detect vacuum leaks without access to a smoke machine?

All the best.

  • Nice explanation of your "diagnostic thinking" and troubleshooting process.
    – dlu
    Commented Dec 17, 2016 at 21:56
  • Thank you for the detailed answer and recommendations, I will keep investigation following these directions. Much appreciated
    – Florent
    Commented Dec 19, 2016 at 17:59

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