P0171 Code: System Too Lean (Bank 1)
P0171 means the engine computer is adding the maximum amount of fuel it can on bank 1 and the mixture is still too lean: too much air or not enough fuel. The most common causes are a vacuum leak, a dirty mass air flow (MAF) sensor, and low fuel pressure from a weak pump or clogged filter.
What "too lean" means
Gasoline engines run best at about 14.7 parts air to 1 part fuel. The computer constantly adjusts fuel using "fuel trims". When it has to add more than about 25% extra fuel to keep the mixture right, it sets P0171.
If both sides of a V6 or V8 are lean, you will also see P0174 (bank 2). Lean on both banks usually points to something shared, like the MAF sensor, a large vacuum leak or fuel pressure.
Symptoms
- Check engine light (sometimes the only sign)
- Rough or hunting idle
- Hesitation or stumbling on acceleration
- Misfire codes such as P0300 in more severe cases
- Hard starting
Common causes
| Cause | Clues | Cost level |
|---|---|---|
| Vacuum leak (cracked hose, intake gasket, PCV valve) | Hissing, worse at idle, fuel trims improve at higher rpm | Low |
| Dirty MAF sensor | Fuel trims high at all speeds; often fixed by cleaning | Low |
| Failing MAF sensor | Cleaning does not help; implausible airflow readings | MAF sensor replacement |
| Low fuel pressure | Lean worse under load or at high speed | Fuel pump or filter |
| Clogged fuel injectors | Lean on one bank, misfires | Medium |
| Exhaust leak before the upstream O2 sensor | Outside air fools the sensor into reading lean | Low to medium |
How to diagnose P0171
- Check fuel trims at idle and at 2,500 rpm: if they improve at higher rpm, suspect a vacuum leak; if they stay high or get worse, suspect MAF or fuel delivery
- Inspect intake hoses, the PCV hose and the intake boot after the MAF sensor for cracks
- Smoke-test the intake for hidden leaks
- Clean the MAF sensor with dedicated MAF cleaner and recheck
- Measure fuel pressure under load
Can you drive with P0171?
Short term, usually yes. Over time, a lean engine runs hotter, can misfire, and may damage the catalytic converter or, in extreme cases, pistons and valves. Address it within days or weeks, not months.
Frequently asked questions
Can a dirty air filter cause P0171?
Rarely. A clogged air filter restricts air, which tends to make the mixture richer, not leaner. An oiled filter can contaminate the MAF sensor, though.
Can a bad oxygen sensor cause P0171?
Occasionally, but most of the time the oxygen sensor is correctly reporting a real lean condition. Look for vacuum leaks and MAF problems first.
Will a fuel additive fix P0171?
Only if the cause is slightly dirty injectors. It will not fix a vacuum leak, a bad MAF sensor or a weak fuel pump.
Related repair costs
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Sources and methodology
Estimates combine typical book labor times, current aftermarket and OEM parts prices, and 2026 US shop labor rates. See exactly how we calculate them.
- U.S. Bureau of Labor Statistics: Occupational Employment and Wage Statistics, automotive service technicians (49-3023), by state
- U.S. Bureau of Labor Statistics: Automotive Service Technicians and Mechanics (pay and duties)
- Federal Trade Commission: Auto Repair Basics (estimates, warranties, your rights)
- NHTSA: Check for recalls before paying for a repair (our free recall check uses NHTSA data)
- ASE (National Institute for Automotive Service Excellence): technician certification