P0171
System Too Lean (Bank 1)
Published · Reviewed and updated
In plain English: The engine computer has run out of room adding fuel to correct a mixture that keeps reading lean on bank one.
A short trip may be possible with normal running and a steady warning light, but arrange prompt diagnosis. Stop for a flashing light, severe rough running, stalling or fuel leakage. The code alone cannot establish that continued driving is safe.
A diagnostic starting point: Unmetered air leak — vacuum hose, intake boot or gasket. Start by asking: Read long term fuel trim at idle, then at twenty five hundred RPM. Illustrative parts budgets span $8–$600 USD, with diagnosis, labor and taxes extra.
Quick answers about P0171
- What does P0171 mean?
- The engine computer has run out of room adding fuel to correct a mixture that keeps reading lean on bank one.
- What should I check first for P0171?
- One starting point is Unmetered air leak — vacuum hose, intake boot or gasket. Air entering downstream of the airflow measurement can disturb the fuel calculation. Confirm the leak with the approved test method. The code alone does not identify a failed part; follow the procedure for your vehicle.
- Can I drive with P0171?
- A short trip may be possible with normal running and a steady warning light, but arrange prompt diagnosis. Stop for a flashing light, severe rough running, stalling or fuel leakage. The code alone cannot establish that continued driving is safe.
- How much does it cost to fix P0171?
- Illustrative parts budgets in this guide span $8–$600 USD. Diagnosis, labor and taxes are extra. Choose a repair only after identifying the actual air, fuel, sensor or circuit fault. These illustrative budgets do not predict which part is needed.
- How do I diagnose P0171?
- Start with: Read long term fuel trim at idle, then at twenty five hundred RPM. Save codes and freeze-frame information. Compare fuel trims under the manufacturer's specified conditions, noting RPM, load, temperature and other bank readings where available. Use the pattern to choose further tests; it does not by itself prove an air leak or weak pump.
Watch the P0171 videos
Written correction, October 15, 2026: The written guide corrects an invalid airflow formula, removes running-engine cleaner-spray advice and clarifies the limits of fuel-trim patterns. The archived videos may contain earlier advice. Use the current written qualifications and exact vehicle procedure.
Full walkthrough — 8:30. Watch on YouTube
What P0171 actually means
The oxygen sensor tells the computer there is too much oxygen left in the exhaust, so the computer adds fuel to compensate. That correction is called fuel trim, and you can read it as a percentage on a compatible scan tool that supports the required live data. Short term trim is the instant correction; long term trim is what the computer has learned it needs all the time.
The computer stores P0171 when bank-one fuel correction or the monitored mixture exceeds its calibrated lean-monitor limit under specified conditions. Thresholds, enabling conditions and sensor types vary; there is no universal fuel-trim percentage that proves this code or its cause.
Symptoms
- Rough or hunting idleWorst at idle, because a fixed-size air leak is a much larger fraction of a small airflow.
- Hesitation or stumble on accelerationEspecially from a stop, when the mixture is furthest from correct.
- Noticeably worse fuel economyCounter-intuitive, but the computer is dumping in extra fuel to chase the reading.
- Hard starting, particularly when coldThe learned trim correction is wrong for a cold start.
- A lean misfire, sometimes with P0300 alongsideIf you also see misfire codes, they are usually a symptom of this, not a separate fault.
Possible causes and what to check
This is a practical diagnostic order, not measured failure rates. Follow the tests and priorities specified for your vehicle; a code alone does not identify a failed part.
Illustrative fuel-trim patterns across engine speed
Illustrative graph, not a recording from your vehicle or a universal service limit. Confirm the test conditions, units and acceptance criteria in the applicable service procedure.
Under the approved test conditions, record short- and long-term fuel trim with engine speed, load and temperature. Compare banks where available. Changing unloaded RPM is not the same as testing under load.
This near-zero line is an illustrative low-correction example. It does not prove that every engine system is healthy; use the manufacturer's ranges and conditions.
Positive correction that decreases as airflow rises can support an unmetered-air hypothesis. Confirm an actual leak and consider other causes rather than identifying a component from the slope alone.
Increasing positive correction can justify fuel-delivery and air-metering checks under the prescribed load conditions. Verify pressure, airflow and sensor feedback before selecting a repair.
How to diagnose it, in order
-
Read long term fuel trim at idle, then at twenty five hundred RPM.Save codes and freeze-frame information. Compare fuel trims under the manufacturer's specified conditions, noting RPM, load, temperature and other bank readings where available. Use the pattern to choose further tests; it does not by itself prove an air leak or weak pump.Then → step 2
-
Is an intake or PCV leak confirmed by the approved test?Inspect the intake and PCV connections. Where specified, use an approved smoke or pressure test with the correct gas, pressure limits, isolation points and engine state. Do not spray flammable cleaner around a running engine. Repair a demonstrated leak and repeat the relevant checks.Yes → Repair the leak, clear, re-driveNo → go to step 3
-
Does measured airflow pass the vehicle-specific checks?Compare airflow and related sensor readings with manufacturer specifications under the stated test conditions. Check intake restrictions, leaks, sensor circuits and engine operating conditions. Low airflow alone does not prove a failed MAF sensor. Clean or replace it only when the approved diagnosis supports that action.Yes → go to step 4No → Diagnose and correct the demonstrated air-metering or circuit fault, then retest
-
Does fuel pressure hold to spec under load?Follow the fuel-system procedure with correctly rated equipment to verify pressure and delivery under the required conditions. High-pressure systems require their specified precautions. A failed reading needs further diagnosis before choosing a pump, filter, regulator or injector repair.Yes → go to step 5No → Repair fuel delivery — filter, pump or regulator
-
Does the upstream mixture sensor pass its specified response test?Identify the fitted sensor type; wideband sensors do not use the same switching interpretation as a narrowband oxygen sensor. Check response, circuits, exhaust leaks and actual mixture using the manufacturer procedure before condemning the sensor.Yes → Re-check for small leaks — something was missedNo → Repair the verified sensor or circuit fault and repeat the monitor checks
Repair the verified cause, repeat the failed checks and confirm the applicable monitor completes.
What you need to do this yourself
- A scan tool that reads live data — Choose vehicle-compatible access to the live data, freeze frame and monitor results required by the diagnostic procedure. Some checks also need bidirectional control.
- Digital multimeter — Use the specified meter and test method for electrical checks. Follow circuit diagrams and avoid applying an unsuitable test load to a computer driver.
- Vehicle-specific service information — Confirm the procedure, safety precautions, tools and acceptance limits for the exact year, model and engine.
Some links may be affiliate links. They cost you nothing extra and do not change which tools are recommended — the diagnostic order on this site is the same either way.
Illustrative repair budgets
These are rough US planning examples, not quotes or measured repair averages. Parts and labor are separate; diagnosis, taxes, difficult access and additional repairs can increase the total.
| Repair | Parts (USD) | Labor time |
|---|---|---|
| Vacuum hose or intake boot Replace the demonstrated leaking or damaged section; access and labor vary. |
$8–$60 | 0.5 h |
| MAF sensor cleaner Use only if the manufacturer allows cleaning and the diagnosis supports it. Follow the specified cleaner and handling procedure. |
$8–$15 | 0.2 h |
| Intake manifold gasket Cheap part, real labour, depending on how buried the manifold is. |
$25–$120 | 2.5 h |
| Mass airflow sensor Use the correct specified part and calibration after the sensor and circuit tests justify replacement. |
$60–$300 | 0.3 h |
| Fuel pump or filter Verify fuel pressure and delivery before choosing a repair; a trim slope alone does not identify the failed part. |
$60–$600 | 2.5 h |
Choose a repair only after identifying the actual air, fuel, sensor or circuit fault. These illustrative budgets do not predict which part is needed.
P0171 video summary · 1:03
A brief introduction to the code and a diagnostic starting point. Use the full guide and vehicle-specific service information for the complete procedure.
Written diagnostic walkthrough
Current written guidance organized by topic. This text may include corrections made after a video was published and is not a verbatim transcript of the archived video.
Meaning and driving guidance
Code P0171. System Too Lean (Bank 1).
In plain English: the engine computer has run out of room adding fuel to correct a mixture that keeps reading lean on bank one.
A short trip may be possible with normal running and a steady warning light, but arrange prompt diagnosis. Stop for a flashing light, severe rough running, stalling or fuel leakage. The code alone cannot establish that continued driving is safe.
How it is measured
Here's what your engine computer is actually measuring.
The oxygen sensor tells the computer there is too much oxygen left in the exhaust, so the computer adds fuel to compensate. That correction is called fuel trim, and you can read it as a percentage on a compatible scan tool that supports the required live data. Short term trim is the instant correction; long term trim is what the computer has learned it needs all the time.
The computer stores P0171 when bank-one fuel correction or the monitored mixture exceeds its calibrated lean-monitor limit under specified conditions. Thresholds, enabling conditions and sensor types vary; there is no universal fuel-trim percentage that proves this code or its cause.
So the code is not a part failure report. It's a performance measurement that came back under spec.
Symptoms
What you'll usually notice, beyond the light itself.
Rough or hunting idle. Worst at idle, because a fixed-size air leak is a much larger fraction of a small airflow.
Hesitation or stumble on acceleration. Especially from a stop, when the mixture is furthest from correct.
Noticeably worse fuel economy. Counter-intuitive, but the computer is dumping in extra fuel to chase the reading.
Hard starting, particularly when cold. The learned trim correction is wrong for a cold start.
A lean misfire, sometimes with P0300 alongside. If you also see misfire codes, they are usually a symptom of this, not a separate fault.
Possible causes
Here is a practical order for checking the possible causes. These are diagnostic priorities, not measured failure rates.
Check 1: Unmetered air leak — vacuum hose, intake boot or gasket. Air entering downstream of the airflow measurement can disturb the fuel calculation. Confirm the leak with the approved test method.
Check 2: Dirty or failing mass airflow sensor. Air-metering errors can bias fuel calculations. Check sensor response and circuits before replacement; clean only if the service procedure allows it.
Check 3: Weak fuel delivery — pump, filter or injectors. The distinguishing sign is that it gets worse under load, not better.
Check 4: PCV system leak or stuck-open valve. A torn PCV hose is just a vacuum leak in a place people forget to look.
Check 5: Exhaust leak ahead of the oxygen sensor. Outside air pulled into the exhaust makes a perfectly good mixture read lean.
Check 6: Failing oxygen sensor reporting falsely lean. Validate sensor response and check for exhaust leaks, wiring faults and actual mixture problems before replacing a sensor.
Use the test results to choose the repair. A position in this list does not prove that a part has failed.
Illustrative diagnostic pattern
This illustrative graph explains a diagnostic pattern. Use the vehicle-specific procedure to interpret it.
Under the approved test conditions, record short- and long-term fuel trim with engine speed, load and temperature. Compare banks where available. Changing unloaded RPM is not the same as testing under load.
This near-zero line is an illustrative low-correction example. It does not prove that every engine system is healthy; use the manufacturer's ranges and conditions.
Positive correction that decreases as airflow rises can support an unmetered-air hypothesis. Confirm an actual leak and consider other causes rather than identifying a component from the slope alone.
Increasing positive correction can justify fuel-delivery and air-metering checks under the prescribed load conditions. Verify pressure, airflow and sensor feedback before selecting a repair.
Diagnostic steps
So here is the order to actually test this, cheapest and most likely first.
Step 1. Read long term fuel trim at idle, then at twenty five hundred RPM. Save codes and freeze-frame information. Compare fuel trims under the manufacturer's specified conditions, noting RPM, load, temperature and other bank readings where available. Use the pattern to choose further tests; it does not by itself prove an air leak or weak pump.
Step 2. Is an intake or PCV leak confirmed by the approved test? Inspect the intake and PCV connections. Where specified, use an approved smoke or pressure test with the correct gas, pressure limits, isolation points and engine state. Do not spray flammable cleaner around a running engine. Repair a demonstrated leak and repeat the relevant checks.
Step 3. Does measured airflow pass the vehicle-specific checks? Compare airflow and related sensor readings with manufacturer specifications under the stated test conditions. Check intake restrictions, leaks, sensor circuits and engine operating conditions. Low airflow alone does not prove a failed MAF sensor. Clean or replace it only when the approved diagnosis supports that action.
Step 4. Does fuel pressure hold to spec under load? Follow the fuel-system procedure with correctly rated equipment to verify pressure and delivery under the required conditions. High-pressure systems require their specified precautions. A failed reading needs further diagnosis before choosing a pump, filter, regulator or injector repair.
Step 5. Does the upstream mixture sensor pass its specified response test? Identify the fitted sensor type; wideband sensors do not use the same switching interpretation as a narrowband oxygen sensor. Check response, circuits, exhaust leaks and actual mixture using the manufacturer procedure before condemning the sensor.
Repair the verified cause, repeat the failed checks and confirm the applicable monitor completes.
Illustrative repair budgets
These are illustrative US parts budgets with labor shown separately. Diagnosis, taxes, access and additional repairs can change the total.
Vacuum hose or intake boot: 8 to 60 dollars in parts, plus about 0.5 hours of labor. Replace the demonstrated leaking or damaged section; access and labor vary.
MAF sensor cleaner: 8 to 15 dollars in parts, plus about 0.2 hours of labor. Use only if the manufacturer allows cleaning and the diagnosis supports it. Follow the specified cleaner and handling procedure.
Intake manifold gasket: 25 to 120 dollars in parts, plus about 2.5 hours of labor. Cheap part, real labour, depending on how buried the manifold is.
Mass airflow sensor: 60 to 300 dollars in parts, plus about 0.3 hours of labor. Use the correct specified part and calibration after the sensor and circuit tests justify replacement.
Fuel pump or filter: 60 to 600 dollars in parts, plus about 2.5 hours of labor. Verify fuel pressure and delivery before choosing a repair; a trim slope alone does not identify the failed part.
Choose a repair only after identifying the actual air, fuel, sensor or circuit fault. These illustrative budgets do not predict which part is needed.
Sources and application limits
These manufacturer documents and technical references support the discussion. A bulletin may apply only to its listed models, engines and production dates; confirm coverage before applying it.
- Mitsubishi P0171 diagnostic procedure — sensor, leak and fuel-system checks
- GM PIE0684 — P0171/P0174 induction-leak investigation on specified 2022 models