System too lean (bank 1)
Long-term fuel trim on bank 1 has run far enough positive that the ECM is adding fuel well past its normal correction range. The engine is getting more air than the computer expects, or less fuel than it commanded.
Long-term fuel trim exceeds the calibrated limit, commonly around +25%, and stays there.
Generic SAE definition. Manufacturers extend and sometimes override these, so the enhanced code on the vehicle in front of you may be narrower than what is written here.
This is the list every other P0171 page will give you. Read it, then read the part underneath it.
Every one of those 6 causes is now a decision somebody in the bay has to make, under a clock, on a car a customer wants back today.
We could mock one up. We’re not going to. A fabricated example of our own product’s output is exactly the kind of thing this page is criticizing. Verdicts appear here as the pilot produces them on real vehicles, with the evidence they actually stood on.
The pilot runs on your existing scan tool and your live repair orders. Bring us a car that's throwing this code.