System too lean — Bank 1
The controller reached a calibrated level of positive fuel correction on Bank 1 while trying to maintain its target mixture.
Conditions that can produce P0171
These are diagnostic branches, not a ranked parts list. Prevalence and testing order depend on the exact vehicle and failure scene.
- Vacuum, crankcase, intake, or charge-air leak
- Low fuel pressure or injector flow
- Under-reported airflow
- Exhaust leak ahead of the feedback sensor
- Purge, sensor, or engine mechanical problem
What to inspect next
- Compare short- and long-term trim by bank and operating state
- Inspect air path and crankcase ventilation
- Verify fuel supply under the condition where correction rises
- Check airflow and feedback signals before replacing the oxygen sensor
Evidence that strengthens the answer
Verify the repair
Preserve the pre-repair record, document exactly what changed, then repeat the relevant operating condition safely. Confirm no current or pending fault returns, review permanent-code and readiness state where applicable, and state which tests or modules remained unavailable.
This page explains a commonly standardized code at a general level. Confirm the exact definition, module, status, test conditions, thresholds, wiring, and service process for the vehicle application using current official information.