What Does P0171 Mean?
The engine is running too lean on bank 1 (or bank A). The ECM has detected that long-term fuel trim has exceeded the positive correction limit, meaning the system is adding more fuel than expected to maintain stoichiometric ratio. This indicates excess air or insufficient fuel is entering the cylinders. The driver may notice rough idle, hesitation on acceleration, possible misfires, and reduced power.
Common Causes
35%
Vacuum leak — cracked intake hose, loose clamp, leaking intake manifold gasket, or torn PCV hose
25%
Dirty, contaminated, or failed mass airflow (MAF) sensor under-reporting air flow
20%
Weak fuel pump, clogged fuel filter, or failing fuel pressure regulator causing low fuel pressure
12%
Leaking or clogged fuel injector(s) on bank 1 delivering insufficient fuel
8%
Exhaust leak before the upstream O2 sensor allowing false air into the exhaust stream
Diagnostic Steps
1
Check short-term and long-term fuel trims at idle and at 2500 RPM. STFT + LTFT combined above +20% confirms a lean condition. If lean primarily at idle, suspect a vacuum leak. If lean at all RPMs, suspect MAF or fuel delivery.
2
Perform a smoke test on the intake system to locate vacuum leaks. Pay special attention to intake manifold gaskets, brake booster hose, PCV valve connections, and any cracked rubber hoses.
3
Clean the MAF sensor with MAF-specific cleaner (not carb cleaner or brake cleaner). Compare MAF grams/second readings to known-good values for the engine at idle (typically 2–7 g/s) and at 2500 RPM.
4
Connect a fuel pressure gauge to the fuel rail. Verify pressure meets spec at idle (typically 35–65 psi depending on the system) and holds steady under load. Check for pressure drop with pump off (should hold for several minutes).
5
If fuel trims are lean on bank 1 only (not bank 2), focus on bank-1-specific causes: individual intake runner leaks, bank 1 injector flow imbalance, or bank 1 exhaust manifold leak.
Estimated Repair Cost
$75 - $800
Parts + labor, varies by vehicle and location
The engine is running too lean on bank 1 (or bank A). The ECM has detected that long-term fuel trim has exceeded the positive correction limit, meaning the system is adding more fuel than expected to maintain stoichiometric ratio. This indicates excess air or insufficient fuel is entering the cylind...
The most common cause of P0171 (Oxygen Sensor System Too Lean Fault Bank A) is: Vacuum leak — cracked intake hose, loose clamp, leaking intake manifold gasket, or torn PCV hose
Typical repair costs for P0171 range from $75 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Drive with caution. A lean condition can cause engine misfires, which may damage the catalytic converter over time. Severe lean conditions at high load can cause detonation (knock), potentially damaging pistons and valves. Avoid heavy acceleration until resolved.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0171 to identify the root cause.
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Quick Info
Category
Powertrain
System
Fuel System
Difficulty
Type
Generic (SAE)
Recommended Tools
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