P0171: System Too Lean (Bank 1)
P0171 is a powertrain diagnostic trouble code in the fuel and air metering group. Our vehicle database stores it against 175 models.
What P0171 actually means
P0171 is one of the most widely logged codes in our database — 175 models record it. It is a powertrain code in the fuel and air metering group, and it is a generic code defined by SAE J2012 and shared by every manufacturer.
The stored definition for P0171 is “System Too Lean (Bank 1)”. The first character of the code identifies the system — Powertrain in this case. The second character tells you who defined the code: this one is a generic code defined by SAE J2012 and shared by every manufacturer. Reading the code that way matters: a generic code tests the same underlying condition on almost every vehicle, so the causes below apply broadly across makes.
Within that system, P0171 sits in the fuel and air metering group. That is the part of the vehicle the control module was watching when it set the code — the sensors and actuators that meter fuel and air into the engine — MAF, MAP, oxygen sensors, and coolant temperature — and it narrows down enormously where a technician will start looking. These codes are settled by comparing live sensor readings against specification, which usually takes a scan tool rather than a parts bin.
What makes P0171 easy to get wrong. A lean code means the engine is drawing more air, or delivering less fuel, than the fuel trims can correct. Two causes account for most cases: unmetered air entering after the mass airflow sensor — a cracked intake boot or a failed PCV hose — and a dirty MAF under-reporting airflow. Both are cheap to inspect, and both are commonly replaced in the wrong order, so the airflow path is measured before parts are bought.
How serious is P0171?
We rate P0171 as Moderate — diagnose soon. That rating comes from the severity recorded in our database for this code, not from how alarming the name sounds.
A moderate-severity code usually leaves the vehicle drivable, but the underlying fault tends to worsen and can cascade into a more expensive repair. It is reasonable to drive cautiously to a workshop in the near term, but the code should be diagnosed rather than cleared and forgotten.
Most likely causes of P0171, ranked
Our database records 28 causes for P0171, ranked by how many of the 175 models that log this code report each one. A cause that shows up across many different platforms is a more probable starting point than one tied to a single model.
- Dirty MAF — recorded on 46 of the 175 models logging P0171.
- Dirty MAF sensor — recorded on 41 of the 175 models logging P0171.
- intake leak — recorded on 34 of the 175 models logging P0171.
- cracked intake boot — recorded on 27 of the 175 models logging P0171.
- PCV valve leak — recorded on 26 of the 175 models logging P0171.
23 further causes are recorded in the database for P0171; the full list is available in the cross-reference.
Causes are listed as recorded in the database. The same code can have different root causes on different vehicles, so the ranking is a starting order for diagnosis rather than a diagnosis in itself.
How a technician diagnoses P0171
Diagnosis for P0171 follows the same logic as any stored powertrain code: confirm the code is active rather than historic, then work through the ranked causes from cheapest and most likely to most intrusive. In practice that means:
How a technician works P0171 — decision path
Powertrain codes respond to data. Capture the freeze frame, then work through the ranked causes from cheapest and most likely to most intrusive.
- Confirm the code is active and capture freeze-frame dataThe snapshot recorded when the code set holds the engine speed, load, coolant temperature and fuel trims at that moment — often the single most useful piece of evidence for separating one candidate cause from another.
- Compare live data against the freeze frameRe-create the conditions from the snapshot and watch the relevant sensors live. A value that cannot reach the expected range narrows the search immediately.
- Work the ranked causes from cheapest and most likelyThe list on this page is ordered by how many of the recorded vehicles show each cause. Start at the top: an intake leak, a dirty sensor or a worn cap is far more common than a failed assembly.
- Repair the confirmed cause, then re-check the monitorsFix what the evidence actually points at rather than replacing parts speculatively — a misfire or lean code will re-set if the real cause is still present.
- Clear the code and complete a full drive cycleClearing alone proves nothing. The code must stay clear through a drive cycle that reproduces the original conditions, and the readiness monitors must run again.
If a workshop replaces a component for P0171 without showing you the measurement that condemned it, ask for the data. On a fuel and air metering code, the recorded causes are usually testable, so a diagnosis should be demonstrable rather than asserted.
What drives the cost of fixing P0171
We deliberately do not print a single repair price for P0171. Most causes in this group are sensors or wiring, which sit at the lower end of the repair-cost range. The honest answer is that the cost is set by which cause is actually present, and that range is wide — a repair for the first cause listed above and the last can differ by more than an order of magnitude in parts alone: a connector repair and a high-pressure pump are not the same job. Add regional labour rates and the spread becomes wider still.
The factors that actually move the number, in rough order of impact:
- Sensor or circuit. A failed sensor and a broken wire can produce the same reading, so the measurement — not the part counter — condemns one or the other.
- Which sensor it is. An oxygen sensor and a mass airflow sensor sit at very different price points, and both live in this group.
- Access. Some MAP and intake-air sensors sit under the intake manifold, which multiplies labour on an otherwise cheap part.
- Diagnosis billed separately. Shops that credit diagnostic time against the repair change the total more than the part choice does.
The practical approach: get the cause confirmed first, then collect two or three written quotes for that specific repair. To judge whether the repair is worth doing against the value of the vehicle, use our repair-or-replace decision guide.
Vehicles in our database that log P0171
This is one of the most widely logged codes in our database — 175 models record it. The makes appearing most often are Honda (15), Chevrolet (13), Nissan (13), Toyota (11), BMW (10). Each model links to its own guide, where this code appears alongside that vehicle's full fault list and running costs.
Frequently asked questions about P0171
What does the P0171 code mean?
P0171 is a powertrain trouble code meaning "System Too Lean (Bank 1)". It belongs to the fuel and air metering group of the SAE J2012 numbering scheme, and it is a generic code defined by SAE J2012 and shared by every manufacturer. The code is set by the powertrain control module when the monitored system reports a value outside its expected range.
Is it safe to drive with P0171?
Severity for P0171 is rated medium in our database. A moderate-severity code usually leaves the vehicle drivable, but the underlying fault tends to worsen and can cascade into a more expensive repair. It is reasonable to drive cautiously to a workshop in the near term, but the code should be diagnosed rather than cleared and forgotten.
What are the most common causes of P0171?
In our vehicle database, the causes recorded for P0171 are: Dirty MAF; Dirty MAF sensor; intake leak; cracked intake boot; PCV valve leak; Carbon-restricted intake runner; leaking PCV; PCV valve; vacuum leak from PCV hose; Intake leak; PCV leak; dirty MAF; PCV valve integrated in valve cover failure; PCV failure; PCV valve failure integrated in valve cover; Carbon-restricted intake; vacuum leak; PCV; Dirty fuel injectors; fuel pressure; Intake boot leak; Intake manifold gasket leak; tune; failing PCV valve; intake carbon; PCV system; PCV valve failure; vacuum leak at intake manifold gasket. They are ranked on the page by how many of the 175 vehicles that log this code list each cause.
How much does it cost to fix P0171?
We do not publish a single figure for P0171 because the real cost depends entirely on which cause is present — a repair for the cheapest cause on the list and the most expensive one differ by an order of magnitude, and labour rates vary by region. The page explains which factors drive the cost and recommends collecting written quotes before authorising work.
Which vehicles commonly report P0171?
Our database stores P0171 against 175 vehicles, most frequently from Honda (15), Chevrolet (13), Nissan (13), Toyota (11), BMW (10). The full list is shown on this page and each model links to its own cost and fault guide.
Related codes and next steps
Codes that share the same system group often appear together, because a single fault can set more than one:
For the full list, see the OBD-II fault code cross-reference. If the repair cost is what you are weighing, model the total with our total cost of ownership calculator before deciding, and check whether an open safety recall covers the fault with the recall lookup.
Sources and scope
Code definitions follow SAE J2012 / ISO 15031-6 naming. The descriptions, listed causes, and severity ratings on this page are the values stored in the VehCalc vehicle database (snapshot 2026-08-20) for the models listed above. Codes and causes vary by model year and engine.
Diagnostic reference only — not repair advice. This page is provided for educational and planning purposes and does not replace an inspection by a qualified technician or the manufacturer's service information. Before repairing a vehicle you depend on, confirm the diagnosis with an ASE-certified mechanic.