What causes a P0171 code?

The most common cause is unmetered air entering after the airflow sensor, usually a vacuum or intake leak or a failed PCV valve. Other frequent causes are a dirty MAF sensor that underreports airflow, low fuel pressure from a weak pump or clogged filter, clogged injectors, and an exhaust leak near the upstream O2 sensor that creates a false lean reading.

How do you fix and clear P0171?

Fix the root cause first, then clear the code. Start with the cheap, high-odds items: clean or replace the MAF sensor, then smoke-test for vacuum and intake leaks and replace any cracked hoses, gaskets, or a failed PCV valve. If those are clean, check fuel pressure and injector flow. After the repair, clear the code with a scan tool or let the ECU relearn fuel trims over a few drive cycles and confirm it does not return.

How much does it cost to fix P0171, and why does it often appear with P0174?

Most P0171 repairs cost between 100 and 350 EUR. A MAF clean can be nearly free, a MAF clean or replacement at a workshop runs roughly 30 to 260 EUR, a vacuum hose or gasket 65 to 260 EUR, a PCV valve or oil separator about 55 to 330 EUR, and a fuel pump can reach 700 EUR with labor. On V engines P0171 (Bank 1) and P0174 (Bank 2) often appear together, which points to a shared cause affecting both banks such as a dirty MAF, low fuel pressure, or a large leak before the throttle body, rather than one isolated cylinder bank.

P0171: System Too Lean (Bank 1)

Quick Answer

Safe to drive (short-term)

P0171 means the air-fuel mixture on Bank 1 (the cylinder bank with cylinder 1) is too lean: too much air or too little fuel. The ECU keeps adding fuel to hit the 14.7:1 target ratio, but it has run out of room to compensate, so long-term fuel trim climbs past about +15% and the light comes on. The single most common cause is unmetered air entering after the airflow sensor, usually a vacuum or intake leak. Most fixes land between 100 and 350 EUR.

Severity: Moderate · Fuel System
Estimated repair cost: From €30 to €700+, varies by vehicle

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What does P0171 mean?

P0171 is a fuel system-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0171 and illuminates the check engine light.

Where it is on the car

Air filter box
Mass airflow sensor (MAF)
Throttle body and throttle position sensor
Intake manifold and MAP sensor
Engine (cylinder head)

A lean code usually means extra air is getting in after the MAF sensor, for example through a leaking intake hose, a cracked vacuum line or a worn manifold gasket. The intake manifold side is the first place to look. A dirty MAF sensor that reads too little air is the other common cause.

What are the symptoms of P0171?

If your vehicle has triggered P0171, you may notice one or more of these symptoms:

Check engine light illuminated (often steady, flashing only if a lean misfire develops)
Rough, high, or surging idle
Hesitation or stumble during acceleration
Reduced fuel economy, because the ECU dumps in extra fuel to chase the correct ratio
Engine may stall at idle or feel like it almost stalls when stopping
Hissing or whistling sound from a vacuum leak under the hood
Occasional lean misfire, which can set P0300 or a cylinder-specific misfire code

What causes P0171?

Here are the most common causes of P0171:

Vacuum or intake air leak (cracked intake hose, failed intake manifold gasket, loose PCV hose, leaking brake booster line), letting unmetered air in that the ECU never accounted for
: high
Dirty or failing Mass Air Flow (MAF) sensor that underreports incoming air, so the ECU injects too little fuel for the air actually entering
: high
Leaking or stuck-open PCV valve or oil separator diaphragm, which acts as a large uncontrolled vacuum leak when it splits
: medium
Low fuel pressure from a weak fuel pump or clogged fuel filter, starving the injectors so the mixture runs lean under demand
: medium
Clogged or partially blocked fuel injectors that cannot flow their rated fuel volume, leaning out the affected bank
: medium
Exhaust leak just upstream of the front O2 sensor, pulling in outside air that makes the sensor read lean even when the mixture is correct
: low

Is it safe to drive with P0171?

Generally yes, for short-term driving. Short-term driving is generally safe, but a lean condition can cause engine overheating and damage if left unaddressed. Avoid heavy acceleration and fix within a week.

How do you diagnose P0171?

Follow these steps to pinpoint the root cause of P0171:

Read short-term and long-term fuel trims with a scan tool. Long-term fuel trim above roughly +15% at idle confirms the lean condition, and if it improves at higher RPM the leak is unmetered air at idle
Smoke-test the intake, or spray brake cleaner or carb cleaner around the intake manifold gasket, vacuum hoses, and PCV connections. A momentary RPM change pinpoints the leak
Inspect and clean the MAF sensor with dedicated MAF cleaner, never touching the sensor element, then recheck fuel trims
Check fuel pressure at the rail with a gauge and compare to the manufacturer specification, watching for pressure drop under load that points to a weak pump or clogged filter
Inspect the PCV valve, oil separator, and their hoses for cracks, splits, or disconnection
Check for an exhaust leak just before the upstream O2 sensor, which can fool the sensor into a false lean reading
Compare Bank 1 and Bank 2 trims on V engines. If both banks read lean, suspect a shared cause such as the MAF, fuel supply, or a leak before the throttle body

How much does P0171 cost to fix?

Repair costs for P0171 vary depending on the root cause and your vehicle. Here are typical estimates:

Repair	Parts Cost	Labor Cost	Total Estimate	DIY Difficulty
Fix vacuum leak (replace hose or gasket)	€5–€60	€60–€200	€65–€260	Moderate
Clean or replace MAF sensor	€10–€200	€20–€60	€30–€260	Easy
Replace PCV valve or oil separator	€15–€150	€40–€180	€55–€330	Moderate
Replace fuel pump	€100–€400	€100–€300	€200–€700	Professional
Clean fuel injectors	€15–€50	€50–€150	€65–€200	Moderate

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

Full repair guides

Engines known for this code

2.0 TFSI

P0171 on specific vehicles

P0171 behaves differently depending on your vehicle. Select your car below for model-specific causes and known issues:

P0171 on Mazda

Mazda CX-5

See Mazda-specific causes & cost

P0171 on Hyundai

Hyundai i30

See Hyundai-specific causes & cost

P0171 on Kia

Kia Ceed

See Kia-specific causes & cost

P0171 on Citroen

Citroen C3

See Citroen-specific causes & cost

P0171 on Opel

Opel Astra

See Opel-specific causes & cost

P0171 on Nissan

Nissan Altima

See Nissan-specific causes & cost

P0171 on Toyota

Toyota Camry

See Toyota-specific causes & cost

P0171 on Honda

Honda Civic

See Honda-specific causes & cost

P0171 on Volkswagen

Volkswagen Golf

See Volkswagen-specific causes & cost

P0171 on BMW

BMW 3 Series

See BMW-specific causes & cost

P0171 on Ford

Ford Focus

See Ford-specific causes & cost

P0171 on Audi

Audi A4

See Audi-specific causes & cost

These codes are often seen alongside P0171 or indicate related issues:

Common questions about P0171

What does P0171 mean?: P0171 means System Too Lean on Bank 1: the air-fuel mixture has too much air or too little fuel. The ideal ratio is 14.7 parts air to 1 part fuel. The ECU keeps adding fuel to correct it, and when long-term fuel trim runs past about +15% without fixing the imbalance, it sets the code. Bank 1 is the side of the engine containing cylinder number 1.
What are the symptoms of P0171?: Common symptoms are a check engine light, rough or surging idle, hesitation on acceleration, worse fuel economy, occasional stalling at idle, and sometimes a hissing vacuum leak under the hood. In bad cases a lean misfire can develop and trigger a P0300 misfire code alongside P0171.
What causes a P0171 code?: The most common cause is unmetered air entering after the airflow sensor, usually a vacuum or intake leak or a failed PCV valve. Other frequent causes are a dirty MAF sensor that underreports airflow, low fuel pressure from a weak pump or clogged filter, clogged injectors, and an exhaust leak near the upstream O2 sensor that creates a false lean reading.
Is it safe to drive with P0171?: For short distances, generally yes, but it is not a code to ignore. A lean mixture raises combustion temperatures and can damage valves, pistons, and the catalytic converter over time, and a lean misfire can leave you stranded. Avoid hard acceleration and aim to diagnose and fix it within about a week.
How do you fix and clear P0171?: Fix the root cause first, then clear the code. Start with the cheap, high-odds items: clean or replace the MAF sensor, then smoke-test for vacuum and intake leaks and replace any cracked hoses, gaskets, or a failed PCV valve. If those are clean, check fuel pressure and injector flow. After the repair, clear the code with a scan tool or let the ECU relearn fuel trims over a few drive cycles and confirm it does not return.
How much does it cost to fix P0171, and why does it often appear with P0174?: Most P0171 repairs cost between 100 and 350 EUR. A MAF clean can be nearly free, a MAF clean or replacement at a workshop runs roughly 30 to 260 EUR, a vacuum hose or gasket 65 to 260 EUR, a PCV valve or oil separator about 55 to 330 EUR, and a fuel pump can reach 700 EUR with labor. On V engines P0171 (Bank 1) and P0174 (Bank 2) often appear together, which points to a shared cause affecting both banks such as a dirty MAF, low fuel pressure, or a large leak before the throttle body, rather than one isolated cylinder bank.

Read our detailed guides

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