P0171 Code – System Too Lean (Bank 1) | Meaning, Causes, Symptoms & Fix

The P0171 diagnostic trouble code indicates that the engine control module (ECM) has detected a lean air-fuel mixture on Bank 1 of the engine.

In simple terms, this means the engine is receiving too much air and not enough fuel.

When P0171 appears, your Check Engine Light will turn on, and the vehicle may begin to run rough, hesitate during acceleration, or consume more fuel than usual.

This is one of the most common OBD2 codes on gasoline engines worldwide.

The code P0171 stands for:

System Too Lean (Bank 1)

A “lean” condition occurs when the engine’s air-fuel ratio contains too much air and not enough fuel.

Modern vehicles use oxygen sensors (O2 sensors) to monitor exhaust gases.
If the sensor detects excess oxygen in the exhaust stream, the ECM assumes the engine is running lean and triggers P0171.

What Is Bank 1?

Bank 1 refers to the side of the engine that contains Cylinder 1.

On inline engines (4-cylinder), there is only one bank — so P0171 applies to the entire engine.

On V-engines (V6, V8), Bank 1 is one side of the engine.

Symptoms of P0171 Code

Common symptoms include:

• Rough idle

• Engine hesitation

• Loss of power

• Engine stalling

• Increased fuel consumption

• Check Engine Light ON

In some cases, the engine may feel weak or unstable during acceleration.

Common Causes of P0171

1. Vacuum Leak (Most Common)

Vacuum leaks allow unmetered air to enter the engine, creating a lean condition.

Common leak sources:

• Cracked vacuum hoses

• Intake manifold gasket

• PCV hose

• Throttle body gasket

2. Dirty or Faulty MAF Sensor

The Mass Air Flow (MAF) sensor measures incoming air.
If it gives incorrect readings, the engine may receive too little fuel.

3. Low Fuel Pressure

Possible reasons:

• Weak fuel pump

• Clogged fuel filter

• Faulty fuel pressure regulator

Low fuel pressure reduces fuel delivery.

4. Exhaust Leak Before O2 Sensor

An exhaust leak can trick the oxygen sensor into detecting extra oxygen.

5. Faulty O2 Sensor

A failing oxygen sensor may send incorrect lean signals to the ECM.

How to Diagnose P0171

Step 1: Check Fuel Trim Data

Using an OBD2 scanner, look at:

• Short Term Fuel Trim (STFT)

• Long Term Fuel Trim (LTFT)

High positive values indicate a lean condition.

Step 2: Perform Smoke Test

A smoke machine helps locate vacuum leaks quickly.

Step 3: Inspect MAF Sensor

Clean it with MAF cleaner spray.

Step 4: Test Fuel Pressure

Compare measured pressure to factory specifications.

Can You Drive With P0171?

Short distances may be possible.

However, driving too long with a lean mixture can:

• Overheat engine

• Cause misfires

• Damage valves

• Reduce performance

Repairs should not be delayed.

Vehicles Commonly Affected

P0171 is commonly found on:

• Toyota

• Chevrolet

• Ford

• BMW

• Audi

• Nissan

• Hyundai

Especially on high-mileage gasoline engines.

FAQ

Is P0171 serious?

Yes. A prolonged lean condition can damage engine components.

Can bad gas cause P0171?

Yes, low-quality fuel can affect combustion.

Can a dirty air filter trigger P0171?

Sometimes, but vacuum leaks and MAF issues are more common.

How long does it take to fix?

Minor repairs can take 1–2 hours.

Final Thoughts

The P0171 code is one of the most frequent OBD2 lean condition codes worldwide.

In most cases, the issue is caused by a vacuum leak or a dirty MAF sensor.

Proper diagnosis using live fuel trim data is essential for accurate repair.

Fixing the problem early prevents engine damage and restores performance.