P0171 Code: System Too Lean (Bank 1)
Bank 1 of the engine is running lean — there is too much air, or too little fuel, in the air-fuel mixture compared with what the computer expects.
What this means for driving
A lean-running engine is usually still drivable in the short term, but it is not something to ignore: running lean over time can cause overheating, misfires, and damage to the catalytic converter. If the engine is stalling, hesitating badly, or running very roughly, have it assessed promptly rather than continuing to drive on it.
When in doubt, have a mechanic assess it promptly. If any of the red-flag symptoms below are present, stop driving and arrange a tow rather than risking it.
Important: This page is general information only — not professional mechanical advice and not a diagnosis for your specific vehicle. Fault causes and severity vary by make, model, year, and condition. A qualified mechanic should confirm any diagnosis before you make driving decisions or authorize repairs.
Red-flag symptoms: stop and get help
- Engine stalling, especially at idle or low speed
- Severe hesitation or loss of power
- Engine overheating
- Check-engine light flashing
Symptoms of P0171
- Check-engine light on
- Rough idle
- Hesitation on acceleration
- Engine stalling
- Poor fuel economy
- Lack of power under load
Most likely causes, ranked
- Vacuum leak — cracked hose, leaking intake gasket, or loose connection
- Dirty or faulty mass airflow (MAF) sensor misreading incoming air
- Weak fuel pump or restricted fuel filter
- Clogged or dirty fuel injectors
- Exhaust leak before the oxygen sensor skewing its readings
- Faulty oxygen sensor
How a mechanic diagnoses it
- Fuel-trim readings checked with a scan tool to confirm the lean condition and how severe it is.
- Intake and vacuum hoses inspected and smoke-tested for leaks.
- MAF sensor cleaned or tested; fuel pressure tested against specification.
Likely fixes
- Repair vacuum leak
- Clean MAF sensor (DIY)
- Replace MAF sensor
- Replace fuel pump
- Fuel injector cleaning or replacement
Repair costs vary by vehicle, region and shop. Get a quote from a local mechanic.
Related codes
- P0174: System Too Lean (Bank 2)
- P0300: Random/Multiple Cylinder Misfire Detected
- P0442: Evaporative Emission Control System Leak Detected (Small Leak)
CheckMyCarCode content is general information only and is not a substitute for professional diagnosis. Trouble-code meanings are standardized for generic (P0xxx) codes, but causes, severity, and repairs vary by vehicle. If your car shows a red-flag symptom, stop driving and get professional help.