System Too Lean (Bank 1)
Fuel System · Air/Fuel Mixture
Bank 1 of your engine is running lean — too much air relative to fuel. The ECU is compensating at its correction limits. Common causes include vacuum leaks, a dirty mass airflow sensor, or a weak fuel pump.
- Vacuum leak (intake manifold gasket, hose)Most common
- Dirty or failing MAF sensorMost common
- Clogged fuel injectorsCommon
- Weak fuel pump or clogged filterCommon
Vehicles most commonly affected by P0171
Extremely common on Toyota Camry and RAV4 (2.4L and 2.5L 4-cylinder), Honda CR-V (1.5L turbo — known oil dilution issue in cold climates), Ford F-150 EcoBoost (intake manifold gasket), and all first-generation Subaru EJ engines. The Toyota 2AZ-FE engine (2002–2011 Camry/RAV4) is particularly prone to P0171 from intake manifold gasket failure.
Real-world P0171 diagnosis scenario
A 2007 Toyota Camry 2.4L with 105,000 miles stores P0171. Long-term fuel trims read +18% at idle and +12% at cruise — both well above the +10% threshold that indicates a real problem. MAF sensor cleaning with $10 of MAF cleaner drops trims to +14% at idle — better, but not fixed. A smoke test identifies a cracked intake manifold gasket leaking unmetered air. Gasket replacement ($200–$350 labor) brings trims to +3%. This is the most common P0171 root cause on the 2AZ-FE engine.
Check your fuel trim numbers before replacing any parts. Long-term fuel trim (LTFT) above +10% at idle confirms a real lean condition. If LTFT is high at idle but normal at cruise RPM, the leak is in the intake manifold (vacuum leak). If LTFT is high at both idle and cruise, the problem is fuel delivery (pump, filter, injectors).
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Frequently asked questions about P0171
What does P0171 mean?
Bank 1 of your engine is running lean — too much air relative to fuel. The ECU is compensating at its correction limits. Common causes include vacuum leaks, a dirty mass airflow sensor, or a weak fuel pump.
Is P0171 serious?
Drive with caution — lean conditions can damage engine over time.
How much does it cost to fix P0171?
Repair costs for P0171 range from $100 – $300 depending on the root cause and your vehicle.
Can I drive with P0171?
You can drive short distances but should have it repaired within 1–2 weeks to prevent further damage.
What is a fuel trim and how do I read it?
Fuel trim is the ECU's real-time correction to the fuel mixture. Long-term fuel trim (LTFT) above +10% means the engine is running lean and the ECU is adding fuel to compensate. You can read fuel trims with any OBD-II scanner or Bluetooth adapter using the Car Scanner app.
Can P0171 cause damage if ignored?
Yes. A sustained lean condition runs the engine hotter than designed, accelerating wear on piston rings and valves. It also causes the ECU to add fuel, reducing efficiency. Fix within 1–2 weeks.
P0171 and P0174 appeared together — what does that mean?
Both banks running lean simultaneously almost always means a dirty MAF sensor or a large intake manifold leak (one that affects both banks). A bank-specific cause like a single bad injector would only trigger one code.