P0171 System Too Lean (Bank 1)

What it means

The front oxygen sensor keeps seeing more oxygen in the exhaust than it should. The computer adds fuel to correct it. That correction is called fuel trim. When the correction reaches its limit, P0171 sets. It means one of three things: air is getting in that the airflow sensor did not measure, the airflow sensor reads too low, or the fuel system is not delivering enough fuel.

The technical version

The fuel system monitor watches short-term and long-term fuel trim. Long-term trim is the learned correction; short-term trim is the fast one. P0171 sets when the total correction on bank 1 stays past the carmaker's lean limit for long enough. On a V6 or V8, P0174 is the same fault on bank 2; both together point to a shared cause.

Official name: Fuel Trim System Lean (Bank 1).

Symptoms

• Often none at first, apart from the check engine light.
• Rough or hunting idle, worse when cold.
• Hesitation or a flat spot when you accelerate.
• A hissing sound from the engine bay (a vacuum leak).
• Misfire codes later, if the mixture gets lean enough.

Causes, most likely first

How likely is our ranking from repair data and common practice, not a published rate.

What the live data shows

Long-term fuel trim on bank 1 is the headline number. Healthy is near 0%. With P0171 it usually sits high and positive. The useful part is where it is worst: high at idle and better at 2,500 rpm points to a vacuum leak, because a fixed leak matters most when little air is flowing. High everywhere, or worse under load, points to the MAF sensor or fuel delivery.

We do not have a recorded drive with P0171 yet, so there is no chart on this page. What the data looks like when the code is real: at a warm idle, long-term fuel trim holds well above 0%, and short-term trim adds more on top. Raise the engine to about 2,500 rpm and hold it. If the total trim drops a lot, suspect a vacuum leak. If it stays high or climbs, look at the MAF reading and fuel pressure. The front oxygen sensor spends more time on the lean side than normal.

What to check first, in order

1. Read codes and freeze frame.

   P0174 as well means a shared cause. Misfire or EVAP codes change the order below.

   What the data shows: The freeze frame shows whether the code set at idle or under load. That points to a leak or to fuel and MAF.

   Cost: no cost if you have a scanner

2. Compare fuel trim at idle and at 2,500 rpm, engine warm.

   This one test splits vacuum leaks from MAF and fuel problems.

   What the data shows: Add short-term and long-term trim. Better at 2,500 rpm: leak. Same or worse: MAF or fuel.

   Cost: no cost if you have a scanner

3. Check the intake: hoses, PCV hose, the boot after the MAF, the oil cap and dipstick seals.

   These are the cheap, common leaks.

   What the data shows: With the engine idling, pinch or cap a suspected hose. If short-term trim falls right away, you found air that should not be there.

   Cost: free to look; hoses are cheap

4. Clean the MAF sensor with MAF cleaner.

   A dirty sensor under-reads airflow on many cars.

   What the data shows: After cleaning, idle MAF should rise a little and the trims should move toward 0%.

   Cost: a can of MAF cleaner

5. Have a smoke test done if you still have not found a leak.

   Smoke shows leaks you cannot hear, such as an intake gasket.

   What the data shows: Not a scanner step. After the repair, trims return toward 0% within a few drives.

   Cost: $683-$834 at a shop (Intake manifold gasket replacement, RepairPal)

6. Test fuel pressure.

   If the trims get worse under load, the fuel system is the suspect.

   What the data shows: Some cars report fuel rail pressure as a live reading. Compare it with the spec for your engine.

   Cost: $1,344-$1,513 at a shop (Fuel pump replacement, RepairPal)

Is it safe to drive?

Get it checked soon

Yes, but fix it within weeks. The engine is still running, but on a correction that is at its limit.

Fine to drive in the short term. A lean engine runs hotter and can start to hesitate, idle rough or misfire, and long, hard driving on a very lean mixture can damage an engine. Fix it within weeks, and before a long trip or towing.

Stop driving and get help if

• The check engine light flashes.
• The engine misfires, stalls or will not idle.
• You smell raw fuel.

What it costs

National averages for the full job at a shop, parts and labor, from RepairPal (updated September 3, 2026). Your price depends on the car and the area. Many of the first checks on this page cost nothing with a scanner. CarMD found the average check engine repair in 2025 was $554, from more than 39 million US vehicles. MAF sensor replacement was CarMD's number 4 check engine repair in 2025, at an average of $346. CarMD averages come from a different method (repairs reported by its users), so they can sit below RepairPal's shop estimates.

Sources for the notes: CarMD 2026 Vehicle Health Index (press release, April 22, 2026).

Can I clear it? Will it come back?

Clear the code after the repair, then drive normally. Watch long-term fuel trim for a few drives: it should settle near 0%. If it climbs again, the leak or fault is still there, and P0171 comes back as a pending code first.

Will it fail an emissions test?

Yes, while the check engine light is on, most OBD emissions tests fail the car.

Carmaker notes

Service bulletins and warranty extensions that name this code. Ask a dealer to check your VIN.

• Hyundai 2014-2015 Tucson 2.4L GDI

  Hyundai bulletin 16-FL-003 (September 2016) is an engine computer update for P0171 and P0420, and says not to replace the catalyst for P0171 alone.

  Hyundai 16-FL-003 via NHTSA static.nhtsa.gov

• Toyota 2009-2018 Sequoia and 2009-2019 Tundra with the 3UR-FBE flex-fuel engine

  Toyota bulletin T-SB-0166-19 (December 2019) covers P0171 and P0174 when the engine estimates more alcohol in the fuel than there is.

  Toyota T-SB-0166-19 via NHTSA static.nhtsa.gov

• Mitsubishi 2014-2016 Outlander 2.4L

  Mitsubishi bulletin TSB-16-13-002 (April 2016) covers a false check engine light at idle in hot weather.

  Mitsubishi TSB-16-13-002 via NHTSA static.nhtsa.gov

Related codes

Also related, no guide yet: P0172, P0101, P0175.

Questions people ask

What is the most common cause of P0171?

A vacuum leak: a cracked hose, a PCV hose, a torn intake boot or an intake gasket. A dirty MAF sensor is next.

Can I drive with P0171?

Yes, for a short time. Fix it within weeks. If the engine misfires or the light flashes, stop driving it hard and get it seen.

Will cleaning the MAF sensor fix P0171?

Sometimes. It is cheap and quick, so it is a good early step. If the fuel trims are only high at idle, a vacuum leak is more likely than the MAF.

Can a bad O2 sensor cause P0171?

Rarely. The front sensor is usually reporting a real lean mixture. An exhaust leak near the sensor can fake it.

What is the difference between P0171 and P0174?

P0171 is bank 1, P0174 is bank 2. Both at once point to something both banks share, such as the MAF, fuel pressure or a large leak.

Sources

1. RepairPal: P0171 code page repairpal.com
2. RepairPal: PCV valve replacement (updated September 3, 2026) repairpal.com
3. RepairPal: Check engine light diagnosis (shop) (updated September 3, 2026) repairpal.com
4. RepairPal: Fuel filter replacement (updated September 3, 2026) repairpal.com
5. RepairPal: Mass airflow (MAF) sensor replacement (updated September 3, 2026) repairpal.com
6. RepairPal: Intake manifold gasket replacement (updated September 3, 2026) repairpal.com
7. RepairPal: Fuel pump replacement (updated September 3, 2026) repairpal.com
8. CarMD 2026 Vehicle Health Index (press release, April 22, 2026) prnewswire.com
9. Hyundai 16-FL-003 via NHTSA static.nhtsa.gov
10. Toyota T-SB-0166-19 via NHTSA static.nhtsa.gov
11. Mitsubishi TSB-16-13-002 via NHTSA static.nhtsa.gov
12. Texas DPS emissions testing FAQ dps.texas.gov
13. Arizona myazcar.com: OBD and the check engine light myazcar.com