P0171 Code: System Too Lean Bank 1, Causes and Fixes

A P0171 code is the OBD-II code for system too lean, bank 1, and that wording sends people looking for a fuel problem. It can be the wrong first move, because air the engine never counted sets the same code.

The code isn't a measurement of how much fuel the engine got. It's the computer announcing it has been adding fuel and has run out of room to add more, and the cheapest way to find out why costs nothing: notice when it runs worst.

What does P0171 mean?

P0171 means system too lean, bank 1. The computer has been adding fuel on that side of the engine and can't add any more.

The engine measures the air coming in and injects what its map says that air needs. Then an oxygen sensor ahead of the catalytic converter reports what was left over after burning, and the computer corrects.

That correction is fuel trim, and long term fuel trim is the number a diagnosis is built on. A large positive long term trim means the computer is adding fuel, and past its limit it stores P0171.

So the code reports the correction, not its cause. The check engine light codes page explains how the code families fit together.

Can you drive with a P0171 code?

In the short term, yes, if it runs smoothly, though a flashing light, stalling, hard hesitation or a fuel smell means stop. Meanwhile it can run the catalyst hotter than intended, which is why this is worth fixing before it becomes a converter.

On a Toyota or Lexus named in recall 20V-012 or 20V-682, run the VIN first. A failing pump there can stall the engine at speed, so smooth running is no guarantee.

Toyota notes that a lean condition may damage the catalyst, and a new converter costs far more than any fix before it. Our P0420 page explains how a converter fails.

What are the symptoms of a P0171 code?

It can be just the light, because the computer keeps correcting the mixture until it reaches its limit. When the mixture runs thin enough to feel, these are the P0171 symptoms carmakers and owners describe:

• A rough idle. GM's coverage for the 1.4-liter says the engine may run rough, especially at idle.
• A noise from the engine. Air drawn through a cracked PCV diaphragm can make a noise, which GM's coverage notice describes.
• Stalling or hesitation. Owners mention stalling in their federal complaints.
• Hard starting. Toyota's fuel pump bulletin lists hard starts with P0171 when the pump loses pressure.

None of these points at one part on its own. When it runs worst narrows it down, and the causes below each name the test that proves them.

What causes a P0171 code?

It comes down to air the engine never counted, or fuel that never arrived. A sensor misreading either one sets the same code, and the carmakers' documents add a software cause no list of parts includes. P0171 alone confirms none of them. The code says the fuel trim hit its limit, not why.

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1. A vacuum leak or a cracked PCV valve

A vacuum leak lets air into the engine after the air flow sensor has done its counting, so the computer meters fuel for less air than arrived. The PCV system, which vents fumes from the crankcase back into the intake, is a common place for it, because its valve and hoses age.

GM found exactly this on its 1.4-liter turbo: a cracked diaphragm in the PCV valve built into the camshaft cover lets in extra air, and it tells technicians to test the valve before replacing the cover. A loose oil filler cap or a dipstick that isn't seated can do the same, and checking both costs nothing.

2. A weak fuel pump, filter or injector

A tired pump, a clogged filter or a dirty injector delivers less fuel than the computer asked for. The air was counted correctly and the fuel fell short, which shows most under load, when the engine wants the most.

Toyota found pumps losing pressure on a long list of its models, with P0171, P0174 or the low-pressure code P0087, and it confirms with a pressure gauge before replacing the pump. Honda found injectors installed wrongly on the Fit, which wear and under-deliver.

3. A misreading air flow or oxygen sensor

A mass air flow sensor that reads low makes the computer meter fuel for less air than is really entering. An exhaust leak ahead of the oxygen sensor does something stranger: fresh air reaching the sensor reads as leftover oxygen, and the computer adds fuel to a mixture that was never wrong.

Nissan warns against replacing the air-fuel sensor first: its revised diagnosis clears the learned values, runs a confirmation procedure, and replaces the sensor only if that procedure sets P2096–P2099. The air flow sensor often sets its own code when it fails, one of P0100–P0104, such as P0101, though not always.

4. Engine software or fuel learning

Sometimes nothing is broken and the computer's logic is wrong, so the fix is a software update. On flex-fuel engines the computer also learns how much ethanol is in the tank, and small fill-ups can confuse it.

Ford reprograms some F-150s for a P0171 with no part fault, and has warned dealers about flex-fuel learning. Toyota's flex-fuel trucks get a reprogram and a reset of the fuel alcohol learning. A dealer can check your VIN for an update before any part is replaced.

A cracked vacuum tee

Is it a vacuum leak or a fuel problem?

A leak is worst at idle and weak fuel is worst under load, and noticing which costs nothing. At idle the engine draws little air, so a fixed leak is a big share of it. Open the throttle and the same leak is too small to matter.

Fuel works the other way. At idle even a tired pump keeps up, but ask for full throttle up a grade and the shortfall appears.

So a car whose trim numbers look ugly at idle and calm on the highway is telling you to look for air. One that's fine around town and lean when worked is telling you to look at fuel, and our page on why a car shuts off at idle covers the idle causes.

How do you fix a P0171 code?

Read the number before anyone names a part. To narrow a P0171 code down, start by reading every code and the long term fuel trim, in this order:

1. Read every code and long term fuel trim. Any scan tool with live data shows it. P0174 beside it, or a P0101 or P0087, changes where you look.
2. Check the cheap seals. The oil filler cap tight, the dipstick fully seated, and no hiss near the engine with it idling.
3. Note when it runs worst. Ugly at idle and calm at speed is air, and fine at idle and lean under load is fuel.
4. Run the VIN. A dealer can check whether GM's, Toyota's or Honda's coverage still applies to your car, any open recall and any software update.
5. Ask for a smoke test or a fuel pressure test. A smoke test finds the leak you can't hear, and a gauge proves the fuel side, as Toyota's bulletin does before replacing a pump.
6. Don't start with the oxygen sensor. Nissan replaces its air-fuel sensor only after its confirmation test sets that sensor's own codes.

Steps 2 and 3 can miss a leak too small to hear or a pump that only falls short under load. If the shop wants a sensor or a converter, ask for the trim numbers and the test behind the quote, and reading a repair estimate shows what else to ask.

How much does a P0171 code cost to fix?

Often little, because the cheapest fixes come first: a loose oil filler cap or a dipstick that isn't seated costs nothing to put right. A smoke test to find a hidden leak costs a $100–$200 fee at an independent shop.

Weak fuel delivery costs more. A new fuel pump runs $504–$555 on a 2019 Toyota Camry 2.5 and $671–$726 on a 2019 Corolla 1.8 at an independent shop, parts and labor.

Our fuel pump cost page prices other cars. The Camry's pump may be free under Toyota recall 20V-012 or 20V-682 and the Corolla's under 20V-012, so check the VIN first.

Does a warranty or recall cover a P0171 repair?

Sometimes, though most of those windows have closed. GM covered a cracked PCV diaphragm on its 1.4-liter for 10 years or 120,000 miles from the date the car was first put in service, and its deadlines to reimburse owners who already paid have passed.

The date the car went into service decides it. Many Toyota fuel pumps with this fault are also in free recalls. Before you pay, run the VIN, the 17-character number on your registration, through our free VIN checker. A dealer can also check the Toyota, Honda and Ford terms.

How is P0171 different from P0174 and P0172?

P0174 is the same lean code on bank 2, and P0172 is the same system in reverse, too rich on bank 1. On an engine with two cylinder heads, each is a bank with its own oxygen sensor and fuel trim, and bank 1 is the side with cylinder 1.

An inline four has one bank and can't set P0174, and most inline sixes can't either, though BMW's split into two banks and can. P0171 with P0174 points at something both banks share: the air coming in, the fuel pump, or a sensor that reads for both. One bank alone points back at that bank's own plumbing. The P0174 page covers the bank 2 code.

P0172 and P0175 mean long term trim reached its limit while taking fuel away. Leaking injectors, too much fuel pressure and an air flow sensor reading high all produce it, and the P0172 page covers it in detail.

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Common questions

Asked most often

Can the fuel itself cause a P0171 code?

It can, mainly on flex-fuel engines, because ethanol carries oxygen inside its own molecule where gasoline's hydrocarbons carry none, and a gallon of it holds about two thirds the energy. More ethanol than the computer expects needs more fuel for the same air, which reads lean until the car learns it.

Will a P0171 code clear itself?

The lamp can, but the code mostly won't: the car turns the lamp off after three sequential drives with similar conditions and no fault, and the stored code stays in memory for around forty warm-up cycles regardless. So have the codes read even after the light has gone out.

What happens if I leave a P0171 code?

The car may feel normal for a while, but it can run the catalyst hotter than intended, and Toyota notes that a lean condition may damage it.

Can an oxygen sensor cause P0171?

Yes, by reporting a mixture leaner than reality, which makes the computer add fuel it didn't need to. An exhaust leak ahead of that sensor does the same thing by letting fresh air reach it.

Prices are ranges compiled in October 2026 for the US market and vary by region, engine and shop rate. Prices from $10 up are rounded to the nearest dollar, so $14.13 reads as $14 and $14.52 as $15. Below $10 they're exact. This article is general information, not advice about your specific vehicle. See the disclaimer.