The most common causes are: Vacuum leak — cracked intake hose, failed intake manifold gasket, or brittle PCV hose (very common); Dirty or failing mass air flow (MAF) sensor under-reporting airflow; Failing or leaking PCV valve; Weak fuel pump or clogged fuel filter causing low fuel pressure; Dirty or partially clogged fuel injectors.

How much does it cost to fix P0171?

Typical repairs range from $10–$30 (DIY) for clean the maf sensor up to $400–$1,000 for replace fuel pump. Costs vary by vehicle and region.

P0171powertrain

P0171 – System Too Lean (Bank 1)

What does P0171 mean?

The engine control module has detected that Bank 1 is running lean — too much air relative to fuel. The computer has added fuel to compensate and hit the limit of its adjustment range, so it sets P0171. This is one of the most common OBD-II codes and is usually caused by unmetered air entering the engine.

What causes P0171?

Listed from most to least common:

1Vacuum leak — cracked intake hose, failed intake manifold gasket, or brittle PCV hose (very common)
2Dirty or failing mass air flow (MAF) sensor under-reporting airflow
3Failing or leaking PCV valve
4Weak fuel pump or clogged fuel filter causing low fuel pressure
5Dirty or partially clogged fuel injectors
6Exhaust leak ahead of the upstream O2 sensor pulling in outside air
7Failing upstream oxygen sensor reporting inaccurate data

Code breakdown

SystemP — Powertrain

TypeGeneric (standardized SAE / OBD-II code — same meaning across most manufacturers)

SubsystemFuel and air metering

Common symptoms

Check engine light on
Rough or unstable idle, sometimes stalling at stops
Hesitation or stumble under acceleration
Reduced power and worse fuel economy
Engine may take slightly longer to start

How to diagnose

1Read live fuel trim data. Positive long-term fuel trim above roughly 10% confirms the engine is compensating for a lean condition.
2Compare fuel trim at idle vs at higher RPM. Lean mainly at idle points toward a vacuum leak; lean across all RPM points toward fuel delivery or the MAF.
3Perform a smoke test on the intake system — this is the fastest, most reliable way to find vacuum leaks.
4Inspect all vacuum hoses, the intake boot between the MAF and throttle body, and the PCV system for cracks.
5Check MAF readings against expected values for your engine, and clean the sensor with MAF-specific cleaner if dirty.
6Test fuel pressure against the manufacturer's specification.
7Inspect the exhaust manifold and pipe ahead of the O2 sensor for leaks.

A persistent lean condition raises combustion temperature and can, over time, damage the catalytic converter, pistons, or valves. Worth addressing promptly.

Common fixes and estimated costs

Repair	Estimated cost
Clean the MAF sensor	$10–$30 (DIY)
Replace cracked vacuum hose or intake boot	$30–$200
Replace PCV valve	$40–$150
Replace intake manifold gasket	$200–$600
Replace MAF sensor	$150–$400
Replace fuel pump	$400–$1,000

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

If P0171 appears together with P0174 (Bank 2 lean), the cause is almost certainly something shared by both banks — a large vacuum leak, the MAF sensor, or fuel pressure — not two separate faults. Start there.

Back to all Powertrain Codes →

This information is provided for general reference only and may vary by manufacturer, model, and year. It is not a substitute for professional diagnosis or repair.