P0171 System Too Lean (Bank 1)
P0171 is a generic OBD2 powertrain code. It means “System Too Lean (Bank 1)” — the same on every make and model, because the definition comes from the SAE J2012 standard rather than from the carmaker.
Ask the AI about P0171 on your car- System
- Powertrain (P)
- Code type
- Generic (SAE)
- Range
- P01xx
- Urgency
- Moderate
What P0171 means
The engine computer keeps adding fuel on bank 1 to hold the ideal air-fuel ratio, and its long-term correction — the fuel trim — has run out of room. Either more air is getting in than the airflow sensor measured, or less fuel is arriving than was commanded. P0171 is bank 1, P0174 bank 2.
Symptoms
- A rough or hunting idle
- Hesitation or stumbling on acceleration
- Misfire codes that may follow (P0300–P0308)
- Worse fuel economy
- Sometimes hard starting
Most likely causes
- A vacuum or intake leak after the airflow sensor — a cracked intake boot, a split PCV hose, a leaking intake-manifold gasket or brake-booster hose
- A dirty mass air flow (MAF) sensor under-reading the airflow
- Low fuel pressure — a weak pump, a clogged filter or a failing pressure regulator
- Clogged injectors
- An exhaust leak ahead of the upstream oxygen sensor
What to check first
- If P0171 and P0174 are both set, look for something shared by both banks — the MAF sensor, fuel pressure or a large vacuum leak; one bank alone points to that side’s intake or injectors
- Listen for hissing at idle and inspect the hoses; a smoke test finds intake leaks in minutes
- Clean the MAF sensor with dedicated cleaner — never touch the element
- Compare fuel trims at idle and at about 2,500 rpm: lean at idle that improves with revs suggests a vacuum leak, lean everywhere suggests fuel delivery or the MAF
Can I keep driving with P0171?
Drivable — plan the repair soon
Short-term, yes. A lean engine runs hotter, and weeks of it can bring misfires, knock and catalyst damage — plan the repair within days to a couple of weeks.
What the wording tells you
Bank 1: engines with two rows of cylinders — V6, V8, boxer — have two banks. Bank 1 is the side that holds cylinder 1, bank 2 the other side; an inline engine has only bank 1.
How P0171 breaks down
Powertrain. Engine, transmission, fuel and emissions — the codes behind the check engine lamp.
Generic code: defined by SAE, the same on every make.
Fuel & air metering.
The specific fault within this range.
Where to look: Fuel & air metering
The sensors the engine computer meters fuel from — mass air flow, manifold pressure, intake air and coolant temperature, throttle position, oxygen sensors and fuel pressure — the fuel trims it learns from them, and the injector, fuel-pump and turbocharger circuits.
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Frequently asked
What does P0171 mean?
P0171 is a generic OBD2 powertrain code. It means “System Too Lean (Bank 1)” — the same on every make and model, because the definition comes from the SAE J2012 standard rather than from the carmaker. The engine computer keeps adding fuel on bank 1 to hold the ideal air-fuel ratio, and its long-term correction — the fuel trim — has run out of room. Either more air is getting in than the airflow sensor measured, or less fuel is arriving than was commanded. P0171 is bank 1, P0174 bank 2.
Can I drive with P0171?
Short-term, yes. A lean engine runs hotter, and weeks of it can bring misfires, knock and catalyst damage — plan the repair within days to a couple of weeks.
What causes P0171?
A vacuum or intake leak after the airflow sensor — a cracked intake boot, a split PCV hose, a leaking intake-manifold gasket or brake-booster hose; A dirty mass air flow (MAF) sensor under-reading the airflow; Low fuel pressure — a weak pump, a clogged filter or a failing pressure regulator; Clogged injectors; An exhaust leak ahead of the upstream oxygen sensor.
How do I clear P0171?
Fix the cause first, then clear it with any OBD2 scanner or app. Once the fault is gone the car also clears it on its own: the check engine lamp goes off after the self-test passes on three drive cycles in a row, and the stored code ages out after about 40 warm-ups. Clearing without a repair only hides it for a while — and resets the readiness monitors an emissions inspection looks at.
Is P0171 the same on every car?
Yes. P0171 is a generic SAE J2012 code, so its definition is identical on every make and model. What differs is the cause behind it and the repair procedure, which depend on the engine — that’s why the assistant on this page asks for your car.
Guides
Definitions: the generic SAE J2012 codes from Brimly’s offline dictionary (data version 2026.09). Causes and advice are general guidance for all makes — the service information for your car has the last word.