P0171
System Too Lean (Bank 1)
The engine computer has detected that bank 1 is running lean, which means the air-fuel mixture has too much air or not enough fuel. This can cause rough running, poor fuel economy, hesitation, or a check engine light. The most common causes are vacuum leaks, a dirty mass air flow sensor, or a fuel delivery problem.
- SEV
- 3/5
- DRIVE
- CAUTION
- DIY
- $20–$250
- SHOP
- $100–$800
Quick answer
AI-CITATION READYWhat it means
Can you drive with it?
Most common causes
- Vacuum leak in intake manifold gaskets, vacuum hoses, PCV hoses, or intake tubing
- Mass air flow sensor dirty, contaminated, or failing
- Plugged fuel filter or weak fuel pump causing low fuel pressure
Typical repair cost
DIY usually runs $20–$250. Typical shop repair lands around $100–$800, depending on the root cause.
01 / Definition
P0171 means the powertrain control module has detected a lean air-fuel condition on bank 1. The fuel trim values have reached a limit trying to correct the mixture, usually because of unmetered air entering the engine, incorrect airflow readings, or insufficient fuel delivery. The fault may be caused by a vacuum leak, contaminated MAF sensor, low fuel pressure, restricted fuel filter, weak pump, dirty injectors, or outdated control module software.
02 / Drive status
With caution. You can usually drive short distances, but the engine may run poorly and fuel economy may drop. If the engine misfires, stalls, or runs very rough, stop driving and repair it soon.
03 / Symptoms
- Check engine light
- Rough idle
- Hesitation on acceleration
- Loss of power
- Poor fuel economy
- Hard starting
- Engine may stall at idle
- Possible surging or stumbling
04 / Causes
| 1 | Vacuum leak in intake manifold gaskets, vacuum hoses, PCV hoses, or intake tubing | high |
|---|---|---|
| 2 | Mass air flow sensor dirty, contaminated, or failing | high |
| 3 | Plugged fuel filter or weak fuel pump causing low fuel pressure | medium |
| 4 | Dirty or plugged fuel injectors | medium |
| 5 | Control module software needs to be updated | low |
05 / Diagnostic sequence
- 01Check for other codes, especially misfire codes, MAF codes, or fuel trim related codes.
- 02Inspect all vacuum hoses, PCV hoses, intake ducts, and intake manifold gaskets for cracks, loose connections, or leaks.
- 03Look at short-term and long-term fuel trims with a scan tool to confirm a lean condition on bank 1.
- 04Check the mass air flow sensor for dirt, oil contamination, or damage, and inspect the air filter and air inlet tract.
- 05Test fuel pressure and fuel volume to rule out a restricted filter or weak fuel pump.
- 06Inspect injector performance and look for uneven injector operation or clogged injectors.
- 07Check for technical service bulletins and verify whether the engine control module needs a software update.
- 08Clear the code after repairs and road test to confirm the fuel trims return to normal.
06 / Repairs
| 1 | Repair or replace leaking vacuum hoses, PCV hoses, intake boots, or intake manifold gaskets | $20–$400 |
|---|---|---|
| 2 | Clean or replace the mass air flow sensor and replace a dirty air filter if needed | $15–$150 |
| 3 | Replace a plugged fuel filter or repair weak fuel pump/fuel delivery issues | $50–$600 |
| 4 | Clean fuel injectors or replace faulty injectors if flow is poor | $50–$500 |
| 5 | Update engine control module software if a manufacturer update is available | $0–$200 |
MFG / Manufacturer notes
These supplements add make-specific diagnostic framing. Pages without full matrix backing or lane approval stay guarded and canonicalize back here until they are explicitly approved for indexing.
07 / Related codes
08 / FAQ
What does P0171 mean?
It means bank 1 is running too lean, so the engine is getting too much air or not enough fuel.
Can a vacuum leak cause P0171?
Yes. Vacuum leaks are one of the most common causes of this code.
Can a dirty MAF sensor cause P0171?
Yes. If the MAF sensor reads air flow incorrectly, the engine computer may reduce fuel too much.
Will P0171 clear itself?
It may clear temporarily if the condition goes away, but it will usually return if the root problem is still present.
09 / Source and method
- DATA BASIS
- OBD-II REFERENCE + OBD2.HELP
- METHOD
- STATIC VALIDATION
- SAFETY
- INFORMATIONAL
This page combines OBD-II diagnostic reference data with OBD2.help generated diagnostic guidance for code meaning, likely causes, and repair direction.
Publishing uses deterministic schema and build validation, plus manual spot checks on representative pages before release.
Safety-critical diagnosis and repairs should be confirmed with a qualified mechanic, especially when the vehicle is misfiring, overheating, or losing power.