P0171 Code (System Too Lean Bank 1): Meaning, Causes and Fix

The P0171 code means the engine isn’t getting the right air-fuel blend (for bank1). Think of the engine like a campfire. You need wood and oxygen at just the right amounts to keep the fire burning nicely. Too much air and the fire burns out. Not enough air and it smolders and smokes.

Same goes for the engine. Inside the combustion chamber, fuel mixes with air and gets lit by the spark plug. If that mixture has too much air and not enough fuel, it burns weak. When that happens, the engine loses power and makes more pollution out the tailpipe.

No good! To keep things running smooth, the engine computer aims for just the right air-fuel mix. The P0171 code switches on when it sees that mixture getting too skinny on fuel (for bank 1).

You can also read my guide on Check engine light on but car runs fine.

How Did I Fix P0171 Code?

My friend drives an older Nissan Altima that would hesitate and misfire when accelerating. I helped him diagnose the issue by connecting a code reader, which revealed a P0171 lean code.

We inspected the engine and found a vacuum hose had become brittle and cracked near the intake manifold. Replacing that hose for $10 fixed the issue and the Altima drove smoothly again.

P0171 Code Detailed Explanation

In the P0171 code, ‘P’ is related to the powertrain of the engine and the first digit i.e. ‘0’ indicates that this trouble code is the same for all engines. A lean air-fuel mixture results in either excess oxygen (air) in the combustion chamber or the fuel is not enough in the combustion chamber to create the right air-fuel ratio.

There are two sides of the engine i.e. bank 1 and bank 2. P0171 trouble code is connected with bank 1 of the engine. Bank 1 is always on the side of cylinder 1, which is always on the front of the engine.

In inline engines, there is only bank 1. While in V-type engines with 6 or 8 cylinders, there is Bank 1 and Bank 2.

You can check out the below video to get an idea of bank 1 and bank 2 in the engine.

Why Is Proper Air-fuel Ratio Necessary In Engine?

The air-fuel ratio is the mix of air and gasoline put together to power up your car’s engine.

The perfect blend is 14.7 parts air to 1 part fuel. But the engine never actually runs at that perfect 14.7 ratio. Instead, it bounces around slightly above and below that number as you drive. When the air side gets bigger than 14.7 parts, that’s called a “lean” mix.

The engine computer uses an O2 sensor to keep tabs on the air-fuel blend. The O2 sensor sniffs the leftover gases after combustion to see if the mix was too lean (too much air) or too rich (too little air). Basically, the O2 sensor measures unburnt oxygen in exhaust gases before entering the catalytic converter.

The mass air flow sensor also helps out. The MAF sensor measures the total air entering the engine. It alerts the computer so it can add just the right amount of fuel to the air stream.

Working together, the O2 sensor and mass air flow sensor try their best to keep your engine running near that perfect 14.7 ratio.

Is P0171 Serious?

P0171 code can become a serious issue if you do not locate the source of this trouble code and fix it. When the air/fuel mixture is too lean, the engine will run rough or stutter.

Due to the improper air/fuel ratio, the engine will not produce enough power while driving. You must first understand what the P0171 code is telling you before you can figure out the problem.

P0171 Symptoms 

Here are some most common symptoms of P0171 code.

• Rough idling
• Loss of power
• Misfire
• Rough acceleration
• Check engine light on
• Hard Starting Condition

Causes of P0171 Code

Here are some most common causes of the P0171 code in engines.

1. Bad Oxygen Sensor

If your car has an error code P0171, it means the O2 sensor before the catalytic converter may not be working properly. This sensor measures the unburnt oxygen in the exhaust to help the ECU know how much fuel to inject.

When the exhaust has little unburnt oxygen, the oxygen sensor sends a message that tells the ECU to put in less fuel. This keeps things balanced.

But a broken O2 sensor can’t give the right info. Then the ECU can’t adjust correctly. That could lead to too much air and not enough fuel. Running too lean is bad for the engine over time.

How to test?

The oxygen sensor is like a little computer that measures how much oxygen is in the exhaust. It tells the engine if it needs more or less gas.

A good oxygen sensor should flip back and forth between high and low voltage every couple seconds – usually between 850 mV and 150 mV. This shows that it’s switching between rich (too much gas) and lean (too little gas).

If your oxygen sensor gets stuck at one voltage or doesn’t flip enough, it could be broken. To test it, attach a voltmeter to the sensor’s wires. If it stays at one voltage or barely moves, it’s probably bad. Replace it so your engine can tune the gas correctly.

The key things the engine needs to know are: Is the mixture right? And is it switching between rich and lean as I adjust? A working oxygen sensor will dance up and down so the computer gets this info. A bad one will flatline.

2. Faulty Mass Airflow Sensor

The Mass Airflow (MAF) Sensor is an important engine component that measures incoming air. It is located in the fresh air intake hose between the air filter and throttle body.

Think of the MAF sensor as a hot plate that needs power to stay hot. More incoming air cools it down more, requiring extra power. It converts these air flow measurements into a square wave signal sent to the engine computer (ECU).

If the MAF sensor fails, the engine computer won’t know the right fuel amounts to inject. Too much or too little fuel can cause driveability issues and trigger the P0171 lean code. A bad MAF confuses the computer on the air volumes entering the engine, throwing off the air-fuel ratio.

The ECU uses the MAF sensor’s grams per second air flow values to balance the fuel properly. So when this sensor is faulty, the engine runs too lean – not enough fuel for the air coming in. Replacing the defective MAF restores the correct air-fuel ratio so the engine runs smoothly again without error codes.

How it becomes bad?

Dirt or dust getting in the MAF (mass airflow) sensor can make it work badly. When the MAF gets dirty, it can cause the idle to get rough.

Try unplugging the MAF sensor while driving to see if performance gets better. If the car speeds up smoothly with the MAF unplugged, the sensor has likely gone bad or needs cleaning.

How to test?

You can check if your MAF is working right by looking at the voltage it makes. Pushing down the gas pedal harder makes more air go through the sensor, which raises the voltage.

When the car is idling or moving slowly, the voltage is usually under 1.0V. But when accelerating, the voltage typically goes up to around 1.7V in the MAF sensor.

How to fix?

To clean the MAF sensor, follow these steps:

• Disconnect the negative terminal of the battery.
  Remove the MAF sensor, it is located on the intake tube between the air filter box and the throttle body facing upwards.
  Spray all of the little sensors you see inside of the MAF carefully with this MAF sensor cleaner.
  Reinstall and reconnect the battery terminal.
• Start the engine and let the ECU relearn idle for 15 minutes.

If still after cleaning the MAF sensor, the graph is not correct, it means you need to replace the MAF sensor. If after replacing the MAF sensor, you are not able to get rid of the P0171 trouble code, it means there is an unmetered air that the MAF sensor is unable to detect. I have explained it in the following sections.

3. PCV System Is Leaking or the Valve Stuck Open

The PCV system is like the lungs of your car engine. It breathes in the gases made by the oil and sends them back to be burned up safely. This keeps these oily gases from polluting the air we breathe.

There’s a hose that connects the PCV valve on the crankcase to the intake manifold. The PCV valve only lets these gases go one way – from the crankcase into the manifold.

If there’s a leak in that PCV hose or the valve gets stuck open, too much air gets pulled into the manifold. This makes the fuel mixture too lean. You’ll usually see the PCV valve on or very close to the intake manifold.

PCV valve is stuck open because gases flowing through your PCV valve leave traces of oil that can clog it up.

4. Vaccum Leaks

A vacuum leak means extra air gets into the engine without being measured by MAF sensor. This throws off the careful balance of air and fuel that makes the engine run smoothly.

To understand vacuums in engines, look at when the piston moves down to let the air-fuel mixture in. This makes a partial vacuum – like a sucking effect. The intake valve opens at the same time, and the vacuum pulls the mixture in from the intake. This is key for combustion later.

When you hit the gas, the throttle opens more to let more air in. This shows why a steady vacuum matters for the best burning.

With leaks, the computer struggles to adjust the fuel properly for the unmeasured air. So the O2 sensor sees the mix as too lean or air-heavy over time. This triggers a fault code. In my case, a cracked hose was the culprit.

Why does it happen?

1. Cracked or Leaking Intake Manifold: The intake manifold evenly distributes air to all the engine cylinders so they can mix fuel and make power. Over time, that manifold can develop cracks or leaks, letting in extra air where it doesn’t belong. This causes a vacuum leak. Checking for damage and replacing a worn intake manifold prevents frustrating vacuum leaks.
2. Worn or Damaged Intake Manifold Gaskets: These keep air from escaping or sneaking in where it shouldn’t. But the gaskets can crack or wear out over time, then air leaks out or leaks in, messing up the fuel mixture. Replacing worn gaskets prevents vacuum leaks.
3. Faulty Vacuum Hoses: There are also little vacuum hoses running here and there around the engine, carrying air between parts. If they get cracks, leaks or come loose, they let in surprise bits of air too, interfering with the smooth running of your engine. Checking those small but important hoses for damage helps avoid vacuum leak issues.

How to detect?

When your engine is running, listen closely. Do you hear a hissing sound, like air escaping from a balloon? That sneaky hissing is often caused by a vacuum leak.

A vacuum leak means there’s a little hole somewhere that air can get sucked into. Not good! The engine needs a tight seal to run its best.

To find the leak, professionals sometimes use smoke machines. They fill up the engine with harmless smoke. Then it’s easy to see where the smoke sneaks out. The smoke shows the hole!

You can also use carb cleaner to detect vacuum leaks.

5. Bad Engine Air Filter

Bad or dirty engine air filter can also cause a P0171 code. If you are using an aftermarket oiled air filter like K&N, it can foul your MAF sensor with oil. Moreover, if the air filter is clogged, it will cause turbulence in the coming air.

The MAF sensor will not be able to determine the correct amount of air entering the engine.

As a result, the engine control computer will reduce the amount of fuel delivered by the injectors so as to maintain the proper air/fuel ratio despite the limited volume of air provided.

6. Clogged Fuel Filter

The fuel filter is responsible for filtering the gasoline, and this helps to keep the engine running smoothly.

If the fuel filter is clogged, enough fuel will not be pumped through the fuel injector. As a result, the mixture will run lean and the ECU will throw the P0171 code.

You can follow the below tutorial to remove the fuel filter from the engine:

To diagnose, whether there is a problem with the fuel filter, you can run a fuel pressure diagnostic test.

Check out the below video to learn how to perform a fuel pressure test using a gauge:

Here’s another test for clogged fuel filter:

Before attempting this test, be sure to drain all gasoline from the fuel filter to avoid any potential hazards.

For the test, you’ll need a straw (to avoid ingesting gasoline) and the fuel filter itself.

1. Slide the straw over the inline connection, carefully ensuring a tight connection that won’t obstruct airflow.
2. Blow into the straw and observe if the air flows through the filter effortlessly or encounters obstruction.

If air flows through the filter without any hindrance, the fuel filter is in good condition. However, if the airflow is slow or blocked, it indicates a faulty fuel filter that needs to be replaced.

7. Bad or Contaminated Fuel Injector

Modern car engines use a high-tech fuel injection system instead of old carburetors. This system is controlled by the engine control module (ECM) computer.

One very common issue that causes the check engine light to show code P0171 for a too lean fuel mixture is a bad fuel injector.

Fuel injectors spray gasoline into the engine to mix with air. Over time, carbon build-up can clog the tiny nozzle holes.

Then the fuel injector cannot properly spray and atomize the fuel. Without a fine mist, the fuel does not mix correctly with air. So the engine runs too lean.

I found this great video to test the fuel injectors.

8. Exhaust Leaks In The Exhaust Manifold Gasket Or Mating Gaskets Before The Heated Oxygen Sensor

Exhaust leaks can cause engine problems. My friend had an error code show up called P0171. He found out it was because of leaks in the exhaust pipe.

It’s kind of tricky to explain how leaks in the exhaust pipe can make the engine lean. The exhaust gases are under high pressure so they shouldn’t let outside air in.

Well, there’s a process called scavenging that happens in car engines. Since I’m a mechanical engineer, I understand this process very well. During combustion, the intake and exhaust valves are both open for a moment.

This helps the exhaust gases leave faster. The fast-moving gases create an area of low pressure, according to venture principle in engineering.

If there’s a leak before the oxygen sensor, outside air gets sucked in. That air brings extra oxygen.

So the oxygen sensor sees higher oxygen levels in the exhaust. It thinks the engine is running lean. Then the error code P0171 shows up.

The symptom of exhaust leaks is that you will hear a whistling noise. You can watch the following video to detect an exhaust leak:

P0171 Code in Nissan, BMW, Ford and Chevy

In Nissan, BMW, Ford and Chevy, the P0171 code is caused by the following reasons:

• Faulty Mass Air Flow (MAF) sensor
• Incorrect Positive Crankcase Ventilation (PCV) hose connection
• Faulty front heated oxygen sensor
• Exhaust leaks
• Dirty fuel injectors
• Clogged fuel filter
• Fuel pump beginning to wear out
• Vacuum leaks downstream of the MAF sensor around the throttle body and or intake manifold