What the numbers actually mean
Fuel trim is the PCM confessing. Short-term fuel trim (STFT) is the correction the PCM is making right now to keep the O2 sensor happy. Long-term fuel trim (LTFT) is the running average it has learned over recent drive cycles.
Positive numbers mean the PCM is adding fuel because the exhaust is running lean. Negative numbers mean it is pulling fuel because the exhaust is rich. A healthy engine at operating temperature idles within roughly plus or minus 3% on both banks.
The idle vs cruise test
This is the single most useful diagnostic on a scan tool. Watch trims at idle, then at 2500 RPM steady, then at highway load. Where the trim shifts tells you what kind of fault it is.
Lean at idle, normal at cruise = vacuum leak. Airflow past the MAF becomes insignificant at load. Lean everywhere = low fuel pressure, clogged filter, weak pump, or dirty injectors. Rich everywhere = leaking injector, high fuel pressure, or a saturated evap canister dumping fuel vapor. Lean bank 1 only, normal bank 2 = a bank-specific leak (intake gasket, PCV routing, upstream O2 sensor drift).
Why chasing codes wastes time
A P0171 System Too Lean code does not tell you the cause — it tells you what the PCM concluded after fuel trim maxed out. Reading trims and freeze-frame data will get you to the actual failed component 90% of the time. The code just points at the neighborhood.
When trims lie
A worn or biased upstream O2 sensor will feed the PCM garbage and the trims become theater. Verify with a graphing scan tool: a healthy O2 sensor swings from about 0.1V to 0.9V roughly once per second at idle. A lazy sensor that hangs at 0.45V is not reporting reality, and no amount of parts throwing will fix trims driven by a bad sensor.