P0171
Medium SeveritySystem Too Lean (Bank 1)
Fuel System
P0171 — Instant Answer
System Too Lean (Bank 1)
Severity: MEDIUM SEVERITY
Meaning: The engine is running lean on bank 1 — too much air or not enough fuel. The ECM has maxed out its fuel trim trying to compensate. Common causes include vacuum leaks and dirty MAF sensors.
Safe to drive? Usually yes with caution, but confirm root cause before long trips.
Estimated repair cost: $0 - $150
Most common causes:
- 1. Vacuum leak (intake manifold gasket, hose, PCV) (35-45% of cases)
- 2. Dirty or failed MAF sensor (35-45% of cases)
- 3. Weak fuel pump or clogged filter (10-25% of cases)
- 4. Leaking fuel injector (10-25% of cases)
Symptoms
- ⚠️Rough idle
- ⚠️Hesitation on acceleration
- ⚠️Poor fuel economy
- ⚠️Possible misfire at idle
Related Symptoms
Common Causes
How to Diagnose P0171
- 1Check for vacuum leaks with smoke test or carb cleaner
- 2Clean MAF sensor with MAF cleaner
- 3Check fuel pressure
- 4Inspect intake manifold gaskets
OEM Procedures for P0171
System Too Lean (Bank 1) Found 58 OEM service manual procedures.
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
11 51 000 Removing and installing/replacing the coolant pump (X5 M) - Replacement: Prepare the coolant pump
Showing 8 of 58 procedures
OEM Diagnostic Flowchart
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Related Resources
Repair guides, symptom pages, and wiring diagrams related to P0171.
OEM Manual Evidence
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Related Symptoms
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Related Codes
Same as P0171 but on bank 2. If both P0171 and P0174 are present, the issue is likely shared (MAF, fuel pump, or large vacuum leak).
Bank 1 is running rich — too much fuel relative to air. The ECM cannot trim enough fuel out to compensate.
The MAF sensor is reading outside the expected range for current engine conditions. Could be dirty, defective, or have an air leak after the sensor.
Frequently Asked Questions
- Can I drive with P0171?
- You can drive but the engine runs lean, which causes poor performance and can damage the catalytic converter over time.
- What is the most common cause of P0171?
- A vacuum leak (cracked hose, loose clamp, or leaking intake gasket) is the #1 cause, followed by a dirty MAF sensor.
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Author & Review
Written by AllOEMManuals Editorial Team. Reviewed by ASE-certified technicians and updated with factory-reference checks for fitment-sensitive steps.
This guide was drafted with AI assistance and reviewed for accuracy by a human editor.
Last updated: 2026-05-07