System too rich — Bank 1
The controller reached a calibrated level of negative fuel correction on Bank 1 while trying to maintain its target mixture.
Conditions that can produce P0172
These are diagnostic branches, not a ranked parts list. Prevalence and testing order depend on the exact vehicle and failure scene.
- Leaking or over-delivering injector
- Fuel pressure above specification
- Airflow signal biased high
- Purge or fuel-vapor control problem
- Feedback sensor, temperature input, or mechanical fault
What to inspect next
- Compare bank trims across idle, RPM, and load
- Check fuel pressure and leak-down using approved procedures
- Review airflow, temperature, purge, and feedback data
- Inspect oil for fuel dilution and address fire or contamination risk
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.