What does P0171 mean?
System Too Lean (Bank 1)
Direct Answer
P0171 means the engine control module (ECM) has detected that the air-fuel mixture in Bank 1 is too lean — there is too much air or too little fuel. The ECM tries to compensate by adding fuel, but when correction limits are exceeded, it sets this code and turns on the check engine light.
Common Symptoms
- ✓Check engine light on
- ✓Rough idle or engine surging
- ✓Hesitation or stumbling during acceleration
- ✓Poor fuel economy
- ✓Engine may run hotter than normal
- ✓Possible misfires (may also set P0300)
Common Causes
- 01Vacuum leak (cracked hose, intake manifold gasket)
- 02Dirty or faulty mass airflow (MAF) sensor
- 03Weak or failing fuel pump
- 04Clogged fuel injectors
- 05Faulty oxygen sensor (upstream)
- 06Clogged fuel filter
- 07PCV valve failure
How to Fix P0171
Check for vacuum leaks
Inspect all vacuum hoses, intake boots, and the intake manifold gasket for cracks or loose connections. Use carburetor cleaner spray near joints while the engine idles — a change in RPM reveals a leak.
Clean or replace the MAF sensor
Remove the MAF sensor and spray it with dedicated MAF cleaner. Let it dry fully before reinstalling. If cleaning does not resolve the code, test the sensor output with a scan tool.
Check fuel pressure
Connect a fuel pressure gauge to the Schrader valve on the fuel rail. Pressure should hold steady at your vehicle spec (typically 40–65 psi). Low pressure points to a weak pump or clogged filter.
Inspect and clean fuel injectors
A fuel injector cleaning service (professional or DIY with a kit) can restore proper spray patterns in clogged injectors.
Replace the oxygen sensor if faulty
An upstream O2 sensor that reads lean continuously may itself be faulty. Verify with live data before replacing.
Frequently Asked Questions
Can a dirty air filter cause P0171?
A dirty air filter alone rarely causes P0171, but a dirty MAF sensor (often caused by a missing or damaged air filter) frequently does. Replace your air filter and clean the MAF sensor.
Will P0171 fail an emissions test?
Yes. An active P0171 with the check engine light on will cause an immediate fail in most states. Fix the root cause and drive 50–100 miles to complete the OBD-II readiness monitors before retesting.
Can a bad gas cap cause P0171?
No. A bad gas cap causes EVAP codes (P0440, P0442, P0455). P0171 is specifically a fuel trim / air-fuel ratio issue.
Estimated Repair Cost
$50–$800
Vacuum hose repair: $50–$150. MAF sensor: $100–$300. Fuel pump: $300–$800.
Safe to Drive?
Yes — with caution
You can drive carefully for a short time, but do not ignore it — lean conditions can cause misfires and long-term engine damage.
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