What does P0171 mean?
The computer constantly adjusts how much fuel it adds based on the oxygen sensors. This code means it has reached the limit of its correction on bank 1.
A lean code means there is more air than fuel in the mix. Either extra air is getting in that the MAF did not measure, or not enough fuel is being delivered.
Codes are a starting point. They tell you which test failed, not which part to replace. Check for other stored codes and fix those in a sensible order.
Symptoms
- Check engine light
- Rough or unstable idle
- Hesitation when accelerating
- Lower fuel economy
- Sometimes misfire codes too
Common causes
- Vacuum leak (cracked hose, intake gasket, PCV valve or hose)
- Dirty or failing MAF sensor reading low
- Low fuel pressure (weak pump, clogged filter, bad regulator)
- Clogged or leaking fuel injectors
- Exhaust leak ahead of the upstream O2 sensor
How to diagnose P0171
- Look at long-term fuel trim. Readings above about +10% at idle point to a lean condition.
- If trims are high at idle and improve at 2,500 rpm, suspect a vacuum leak. If they get worse at higher rpm, suspect fuel delivery or the MAF.
- Smoke-test the intake for leaks.
- Check fuel pressure against spec.
- Clean the MAF sensor and recheck.
How to fix it
- Repair vacuum or intake leaks
- Clean or replace the MAF sensor
- Replace the PCV valve or hose
- Repair fuel pressure problems (pump, filter, regulator)
- Clean or replace injectors
Estimated repair cost
Likely repairs for P0171 run from about $20 to $1,300, depending on the cause.
| Repair | Typical range (parts + labor) |
|---|---|
| Diagnostic fee (scan + testing) | $100 - $200 |
| Vacuum leak repair (hose/gasket) | $100 - $450 |
| PCV valve / hose replacement | $50 - $300 |
| MAF sensor cleaning | $20 - $100 |
| Mass airflow sensor replacement | $150 - $450 |
| In-tank fuel pump (gasoline) | $500 - $1,300 |
These are rough US estimates for an independent shop in 2026, based on typical parts prices and standard labor times. Dealers, big trucks, luxury brands and high-cost areas often run higher. They are not quotes. Use the repair cost estimator to adjust for your vehicle.
How serious is P0171?
Moderate severity
You can usually drive it short term, but a lean engine can misfire and run hot, so fix it soon.
Make-specific notes
Ford
Cracked PCV hoses and intake leaks are common causes on many Ford engines, along with dirty MAF sensors.
Toyota
A dirty MAF sensor and a failing air-fuel ratio sensor are frequent causes.
VW / Audi (2.0T and others)
A failed PCV valve or diaphragm in the valve cover is a well-known cause of lean codes and a whistling noise.
BMW
Crankcase vent (PCV) valve failures, cracked vacuum lines, and leaking intake boots are classic causes.
Always check open recalls and technical service bulletins for your VIN.
P0171 FAQ
Can a bad O2 sensor cause P0171?
It can, but most lean codes come from a vacuum leak, dirty MAF sensor or low fuel delivery. The upstream oxygen sensor is often just reporting the real problem.
Can I drive with P0171?
Usually for a short time. A lean engine can misfire and run hotter, so do not ignore it.
What does P0171 mean?
P0171 stands for "System Too Lean (Bank 1)". The computer constantly adjusts how much fuel it adds based on the oxygen sensors.
How much does it cost to fix P0171?
It depends on the cause. The likely repairs for P0171 range from about $20 to $1,300 at an independent shop in the US, plus a diagnostic fee of roughly $100-$200. These are rough estimates; dealer prices and some vehicles run higher.
What is the most common cause of P0171?
The most common causes are: vacuum leak (cracked hose, intake gasket, PCV valve or hose); dirty or failing MAF sensor reading low; low fuel pressure (weak pump, clogged filter, bad regulator).