P0171 System Too Lean (Bank 1)
The engine is getting too much air or too little fuel on bank 1, and the computer has reached the limit of how much extra fuel it can add to correct it.
Common causes
- Vacuum leak (cracked hose, intake gasket, PCV)
- Dirty or faulty mass airflow (MAF) sensor
- Weak fuel pump or clogged fuel filter
- Faulty upstream oxygen sensor
Symptoms
- Rough idle, hesitation, poor acceleration – or no symptoms at all
Can I drive with it?
Usually drivable, but a lean engine runs hotter and can misfire. Get it checked soon.
Tip: If P0171 and P0174 appear together, the cause is shared by both banks – think MAF sensor, vacuum leak or fuel pressure.
Which scanner reads it?
Any OBD2 scanner reads it. Live fuel-trim data (short- and long-term) is the key to diagnosing it – look for a tool that shows it.
Budget tools like the ANCEL AD310 read it; tools with live data and graphing – like the BlueDriver or OBDLink MX+ – help you find the cause. Not sure which fits you? Try the Find My Scanner quiz.
In the data
How common is it? At New York’s emissions inspections in 2020–21, P0171 was the #2 code found on cars that failed – 4.7% of all the codes found.
Of the vehicle complaints owners filed with NHTSA from 2015 to 2025, 87 quote P0171 – most often from Chevrolet, Ford, Toyota owners. Complaints lean toward serious problems, so treat this as a sign of how often the code turns up in real reports, not how often cars get it.
Both sources are compared on the Trends page.
Related codes
General information to help you understand a code – not a diagnosis. Your car’s service information or a qualified mechanic has the final word. See how we research.