P0171 Code: System Too Lean (Bank 1)

What does P0171 mean?

P0171 means the engine is running lean on bank 1. "Lean" means the air and fuel mixture has more air than the computer wants, and "bank 1" is the side of the engine that holds cylinder 1. On a four-cylinder engine there is only one bank, so P0171 covers the whole engine.

The engine computer, often called the PCM or ECM, constantly fine-tunes the mixture. It measures incoming air with the MAF sensor (or a manifold pressure sensor on some engines), then reads the exhaust with the upstream oxygen sensor. If the oxygen sensor reports a lean exhaust, the computer adds fuel. These corrections are called fuel trims. When the computer has added as much fuel as it is allowed and the exhaust still reads lean, it stores P0171 and turns on the check engine light.

ALLDATA DIY puts it simply: P0171 indicates too much air, not enough fuel, or both ^[1]. The oxygen sensor that reports the lean condition is usually the messenger, not the culprit.

Is it safe to drive with P0171?

Usually yes, for a short time, but treat P0171 as "fix soon." Most cars with this code still start and drive, and you may notice nothing at all. Others show a rough or hunting idle, hesitation when you press the gas, or weak power under load. Carista lists rough idle, possible stalling, hesitation, reduced power and idle misfires as typical symptoms ^[2].

The risk grows the longer you ignore it. A lean mixture burns hotter and can lead to misfires. The California Air Resources Board says most manufacturers advise service as soon as conveniently possible when the light comes on, and that continued driving can damage other parts ^[3].

If the check engine light starts flashing, or the engine shakes and stumbles, pull over when it is safe and stop driving. A flashing light means a misfire is happening that could damage the catalytic converter, according to the same CARB fact sheet. If P0171 shows up with misfire codes like P0300, read our guide to a flashing check engine light before you drive any farther.

What causes P0171?

P0171 almost always comes down to one of three things: extra air the computer did not measure, too little fuel, or a sensor that gives the computer the wrong picture. Here are the usual suspects, most common first.

1. Vacuum or intake air leak after the MAF sensor. A cracked intake boot, a split vacuum hose, a torn PCV hose or valve, or a leaking intake manifold gasket. ALLDATA DIY and Carista both list vacuum leaks first ^[1].
2. Dirty or failing MAF sensor. If the sensor under-reports airflow, the computer injects too little fuel.
3. Low fuel pressure. A weak fuel pump or a clogged fuel filter starves the engine, often worst under load or at high speed.
4. Restricted or leaking fuel injector. A clogged injector on bank 1 delivers less fuel than commanded.
5. Faulty upstream oxygen sensor. Possible, but less common than people think.
6. Exhaust leak ahead of the oxygen sensor. Outside air pulled in near the sensor can make the exhaust look leaner than it is.

Cause	Typical clue	Cheapest first step
Vacuum or intake leak	Worse at idle, hissing sound, high trims at idle that drop at higher rpm	Look and listen around hoses and the intake boot
Dirty MAF sensor	Lean at idle and under load, may set with airflow codes such as P0101	Inspect the air filter and sensor, clean with MAF-safe cleaner
Low fuel pressure	Hesitation under load, worse uphill or at highway speed	Have fuel pressure tested
Injector problem	Often with a misfire on one cylinder	Scanner data, then a technician test
O2 sensor	Slow or stuck readings in live data	Check it last

If your engine has two banks and both P0171 and P0174 are set, look for something both sides share, such as a MAF sensor, fuel pressure, or a large intake leak. P0171 alone on a V6 or V8 points more toward something on bank 1 only.

How do you diagnose P0171?

Work from cheap and safe to expensive. You need a scanner that shows freeze frame and live data; our guide on how to use an OBD2 scanner covers the basics.

1. Read every code and the freeze frame. Write down engine speed, load, coolant temperature and fuel trims at the moment P0171 set. A lean code that set at idle points toward a vacuum leak. One that set at highway speed points toward fuel delivery.
2. Check fuel trims in live data. Positive numbers mean the computer is adding fuel. Carista notes that short and long term trims above +10% help confirm a real lean condition ^[2]. Compare trims at idle and at about 2,500 rpm. If they fall back toward zero at higher rpm, suspect a vacuum leak.
3. Do a visual check with the engine off. Look at the rubber intake boot between the air filter box and the throttle body, especially the folds underneath. Check vacuum and PCV hoses for cracks or loose ends. Make sure the oil filler cap is tight and the dipstick is fully seated, since both seal the crankcase.
4. Listen with the engine idling. A hiss or whistle often gives away a leak. Keep hands, hair and clothing away from belts and the fan.
5. Inspect the MAF sensor and air filter. A soaked or very dirty filter, or debris on the sensor, can skew airflow readings.
6. Smoke test and fuel pressure test. A shop smoke test finds leaks you cannot see. Fuel pressure testing involves a pressurized fuel system, so have a qualified technician do it.
7. Look at the oxygen sensor last. ALLDATA DIY warns that replacing the oxygen sensor first is a common mistake, because the sensor is often just reporting a real lean condition.

How do you fix P0171, and what does it cost?

The fix is whatever the diagnosis found, so the cost depends entirely on the cause. A loose hose clamp or a reseated oil cap costs nothing. A cracked intake boot or vacuum hose is usually an inexpensive part. Cleaning a MAF sensor with the right cleaner is a cheap do-it-yourself job, but replacing the sensor costs more. Fuel pump and intake manifold gasket jobs sit at the high end because of parts and labor time.

We have not found a reliable U.S. repair cost survey for P0171 as of November 2026, so we do not quote a dollar range. These things drive the bill:

• What failed: a hose or boot versus a fuel pump or intake gasket.
• Engine layout: parts buried under the intake manifold or inside the fuel tank take more labor.
• Diagnostic time: shops often charge for a smoke test or fuel pressure test before any repair.
• Warranty: emissions parts may be covered by the federal emissions warranty, and some makers have issued service bulletins for lean codes.

After the repair, clear the code with your scanner and drive normally. Clearing does not fix anything; if the cause is still there, P0171 comes back. Also know that clearing codes or disconnecting the battery resets the readiness monitors. California now requires all readiness monitors to be set to pass a Smog Check, and monitors complete through normal driving or a specified drive cycle ^[4]. Fix the cause first, then let the car finish its self-tests before any inspection.

Which vehicles get P0171 most often?

Any gasoline car can set P0171, but some engines have known, documented patterns. On several GM 1.4L turbo models, including the Chevrolet Cruze, Sonic and Trax, GM bulletin 19-NA-114 links P0171 to a torn PCV diaphragm in the camshaft cover ^[5]. See our P0171 on Chevrolet page.

Toyota bulletin T-SB-0166-19 covers flex-fuel 5.7L Tundra and Sequoia trucks that set lean or rich codes ^[6]. Our P0171 on Toyota page explains it. If you see the opposite code, read about P0172, system too rich.