P0171 — System too lean (bank 1)
Generic powertrain DTC · MEDIUM priority for follow-up (not a certified safety rating)
What the code means
Long-term fuel correction indicates an air-fuel mixture leaner than expected on bank 1.
What you might notice
Rough idle; hesitation; poor acceleration; check-engine light.
Possible causes — not a diagnosis
Unmetered intake air; low fuel pressure; mass airflow measurement issue; exhaust leak before sensor.
Useful diagnostic checks
Compare short/long fuel trims at idle and under load; check intake hoses, pressure and airflow measurements.
Next steps
Find the cause of unmetered air or inadequate fuel rather than assuming the oxygen sensor failed.
Related OBD-II codes
These generic definitions are explanatory summaries of commonly standardized OBD-II powertrain conditions. Vehicle-specific code logic and service procedures must be verified against manufacturer documentation. EPA: On-board diagnostics.