P0171 - System Too Lean Bank 1
P0171 is a generic powertrain diagnostic trouble code indicating System Too Lean Bank 1. It relates to the Fuel & Air Metering system.
DTC Breakdown (SAE J2012)
| System | Powertrain (P) |
| Type | Generic |
| Subsystem | Fuel & Air Metering |
| Fault Index | 71 |
| Standard | SAE J2012 / ISO 15031-6 |
What does diagnostic code P0171 mean?
The engine control module has detected that the fuel-to-air ratio is too lean on Bank 1, meaning there is too much air or not enough fuel. The computer has reached its limit trying to add fuel to compensate.
Severity Level & Driveability Details
Severity: Moderate
Prolonged driving with a lean engine can cause internal cylinder overheating, potentially burning exhaust valves, fouling spark plugs, or damaging pistons under load.
Common Symptoms of P0171
- Rough engine idle or hunting idle speeds.
- Engine hesitation or stumbling on acceleration.
- Hard starting, especially when the engine is cold.
- Mild engine misfires and loss of power.
- The check engine light will activate due to lean condition on Bank 1.
Typical Causes for P0171
- Vacuum leaks (cracked PCV hoses, leaking intake manifold gaskets, disconnected vacuum lines).
- Dirty or failing Mass Air Flow (MAF) sensor reporting low airflow.
- Low fuel pressure (weak fuel pump, clogged fuel filter).
- Failing fuel pressure regulator or leaking fuel injector.
Diagnostic and Troubleshooting Steps
Technicians look at fuel trims on a scanner. If long-term fuel trim is high (+15% or more) at idle but drops as engine speed increases, it is a vacuum leak. A smoke test is used to locate the vacuum leak. Clean and test the MAF sensor readings.
Typical Repairs and Fixes
- Replacing leaking intake gaskets, PCV hoses, or vacuum lines.
- Cleaning or replacing the Mass Air Flow (MAF) sensor for Bank 1.
- Replacing the fuel pump or clogged fuel filter.
- Replacing a failed fuel pressure regulator.
Sensor Location & Parts Involved
The intake gaskets and vacuum lines are located on top of the engine block near the intake manifold on Bank 1.
Clearing Code & Recurrence
Reset the code using a scan tool and clear fuel trims to verify trims on Bank 1 return to normal.
Emissions & Smog Inspection Impact
Causes elevated cylinder temperatures on Bank 1, leading to high nitrogen oxide (NOx) tailpipe emissions.
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Related Diagnostic Codes
Sources & References
This diagnostic guide is compiled and verified against the following authority sources:
- CARB OBD Regulations Page
- EPA Fuel Trim Reference Guide
Frequently Asked Questions about P0171
What does the Bank 1 fuel trim lean code P0171 mean?
The engine control module has logged P0171 after detecting a lean air-fuel mixture on Bank 1, meaning there is too much air or too little fuel entering the cylinders.
Is it safe to drive with the Bank 1 fuel trim lean code P0171 active?
Yes, but avoid aggressive acceleration or heavy towing when Bank 1 runs lean.
How much does it cost to repair a Bank 1 fuel trim lean code P0171 fault?
Diagnostics for Bank 1 lean conditions cost $100-$150. Intake gaskets on Bank 1 are $250-$500; MAF sensor replacement is $150-$300; fuel pump is $400-$800.
What are the common causes of the Bank 1 fuel trim lean code P0171?
Trigger conditions for the Bank 1 fuel trim lean code P0171 include: Intake vacuum leaks (cracked PCV hoses, leaking manifold gaskets) on Bank 1., Dirty or failing Mass Air Flow (MAF) sensor., Weak fuel pump or clogged fuel filter reducing fuel delivery to Bank 1..
Can I clear the check engine light for Bank 1 fuel trim lean code P0171?
Reset the code using a scan tool and clear fuel trims to verify trims on Bank 1 return to normal.
Will the Bank 1 fuel trim lean code P0171 cause an inspection failure?
Causes elevated cylinder temperatures on Bank 1, leading to high nitrogen oxide (NOx) tailpipe emissions.