What P0171 means
The control system reached a lean-condition diagnostic threshold for Bank 1. The data does not by itself prove a vacuum leak, MAF sensor, or fuel pump.
A trouble code identifies a detected condition. Use the surrounding evidence to choose a test instead of treating the code as a failed-part diagnosis.
- 01PreserveCode, module, freeze frame, readiness
- 02ClassifySystem Too Lean (Bank 1)
- 03InspectUnmetered air or air-measurement error / Fuel delivery or injector concern
- 04TestPreserve freeze frame and both-bank fuel trims / Compare idle, raised-speed, and loaded behavior
- 05VerifyRepair only when evidence supports it; retest
Follow the sequence, stop when evidence changes the path, and verify the exact vehicle information before acting.
Record every stored and pending code, freeze frame, readiness state, and relevant live data before clearing anything. Companion codes and operating conditions often matter more than the code name.
Symptoms you may notice
- Check-engine light
- Rough idle
- Hesitation
- Possible reduced power
Cause groups to investigate
Use codes, data, inspection, and controlled testing to decide whether this group is supported on the exact vehicle.
Use codes, data, inspection, and controlled testing to decide whether this group is supported on the exact vehicle.
Use codes, data, inspection, and controlled testing to decide whether this group is supported on the exact vehicle.
Use codes, data, inspection, and controlled testing to decide whether this group is supported on the exact vehicle.
Diagnostic route
Preserve freeze frame and both-bank fuel trims
Follow the vehicle’s exact test conditions, wiring, thresholds, and safety procedure. Record the result before moving to the next step.
Compare idle, raised-speed, and loaded behavior
Follow the vehicle’s exact test conditions, wiring, thresholds, and safety procedure. Record the result before moving to the next step.
Inspect intake, PCV, exhaust, and recent work
Follow the vehicle’s exact test conditions, wiring, thresholds, and safety procedure. Record the result before moving to the next step.
Test fuel delivery and sensor plausibility as indicated
Follow the vehicle’s exact test conditions, wiring, thresholds, and safety procedure. Record the result before moving to the next step.
Confirm trims after repair
Follow the vehicle’s exact test conditions, wiring, thresholds, and safety procedure. Record the result before moving to the next step.
Common mistakes
- Replacing an oxygen sensor because the system reports leanA diagnostic trouble code identifies a detected condition; it does not automatically identify the failed part.
- Using one fuel-trim value without operating contextA diagnostic trouble code identifies a detected condition; it does not automatically identify the failed part.
Sources and verification
The standardized code meaning is tied to SAE J2012. Additional government sources may document the monitor family or public code description. Cause groups and diagnosis remain general guidance; manufacturer-enhanced definitions, enable criteria, test values, and exact procedures require authoritative information for the vehicle.