There are 3 elements a combustion engine needs in order to work — air, fuel, and spark. A failure in any of these areas can lead to a decrease in performance or your vehicle not starting at all.

When you’re having issues with the air delivery, you might get a P0171 check engine light code on the dash. This code can be triggered by a number of causes, including vacuum. It’s borderline impossible to avoid vacuum-related issues as your vehicle ages. Rubber hoses become brittle and hose clamps rust, both can leave with a car that feels like it’s seen better days.

Fortunately, we have the answer to diagnosing and fixing vacuum pressure issues. This article will focus on the causes, symptoms, and any other problems you might encounter when facing this trouble code.

What Is the P0171 Code, and What Does It Mean?

In the most literal sense, the P0171 code means “System Too Lean (Bank 1).” This trouble code points to a lean running engine condition on the first bank of cylinders. Keep in mind that even inline engines can have two “banks.” 

For example, the first 3 cylinders of an inline-6 are sometimes defined as Bank 1, while the 3 cylinders are Bank 2. This bit of info could help you determine what you’re dealing with during the diagnostic phase. 

What Is a Lean Condition? 

To run an engine, you need to feed it a very specific mix of fuel and air. By adjusting this ratio, you can change how the engine reacts and behaves in different scenarios. The ideal, or “stoichiometric fuel ratio,” is 14.7:1. In other words, for every unit of fuel, you’re adding 14.7 units of air. When the amount of fuel begins to drop, and the air/fuel mix starts moving away from the stoichiometric ratio, you’re dealing with a so-called lean mixture. 

Most engines aren’t built to run on a lean mix. In fact, a lean mixture can cause damage to your pistons since a lean mix forces the engine to run hot. As a result, the risk of detonation is far greater, and detonation can cause damage to the pistons. 

What Causes the P0171 Code?

Many issues can cause a lean running condition, making the process of diagnosing the issue somewhat difficult at times. That being said, knowing some of the most common causes can make tracking down the culprit much easier. Here’s what could be giving you the jeopardy: 

• Fuel system issues
• Vacuum leak
• Failed fuel injectors
• Faulty mass air flow sensor
• Damaged wiring harness

Fuel System Issues

Your fuel system can be a major source of issues. A failed fuel pressure sensor, pump, or clogged fuel filter can cause the P0171 trouble code. Anything that restricts the fuel pressure, or causes an obstruction within the system will prevent the engine from getting the fuel it needs. 

Vacuum Leak

Hose leaks are a common occurrence in older vehicles. With this trouble code, these leaks will most likely be near the air intake. Unfortunately, they can be quite difficult to track down if they are located on one of the secondary vacuum systems. There will be additional trouble codes with the P0171 code when these issues occur, so be on the lookout for those.

Failed Fuel Injectors

Manufacturers design fuel injectors to spray the optimal amount of fuel for their engines. However, even one experiencing issues can cause the air-fuel mixture in a cylinder to become lean. This trouble code points to a failure in the first cylinder bank. If there is a failed injector, it will be there.

Clogged fuel injectors are another point of failure that can cause the P0171 code. Modern direct injection engines expose their injectors to a lot more carbon build-up than port-injected engines did previously. This means that more vehicles are experiencing lean-running conditions than ever before.

Faulty Mass Air Flow Sensor

Although MAF sensors have a specific code that will be logged when they are experiencing issues. The P0171 code is an additional code that may appear on top of a MAF sensor code. 

A failed mass air flow sensor can cause a host of issues, one of the major ones being a lean running condition. This happens when the sensor sends an incorrect air reading to the PCM, which adjusts the fuel injection based on the air intake.

Damaged Wiring Harness

The wiring harness carries voltage to and from the sensors and engine control unit. If either of these processes is disrupted, you can have a host of trouble codes, including the P0171.

Bad O2 Sensor

O2 sensors tell the engine what’s going on with the exhaust gases in terms of their composition. Using that data, the ECU then makes adjustments to different parameters, including the air/fuel mix. If the O2 sensor goes berserk, it could tell the ECU that there is not enough air in the exhaust, causing it to compensate by adding more. Needless to say, depending on how much additional air is added, that can cause a serious lean condition. 

What Are the Common Symptoms of P0171 Fault Code?

Diagnosing your vehicle starts with identifying symptoms. This is a fairly general trouble code, so the symptoms will not tell you much about the specific cause, but will allow you to narrow down the issue to a specific system. Most commonly, these are:

• Performance reduction
• Check engine light
• Stalling while driving
• Bad starting behavior

Performance Reduction

A reduction in performance is a given when you have a lean running engine. Your engine is tuned to run at a specific air-fuel ratio. Anything other than that will cause issues such as jerky acceleration and low power when you step on the accelerator. 

Check Engine Light

The check engine light or malfunction indicator light is the first sign of trouble with your vehicle. Although it will not flash as the P0171 is not one of the most serious trouble codes, it is a vital indicator to notify you, you need to further investigate the source of the problem.

Stalling While Driving

Of course, if combustion can not occur correctly in your engine, it will stall. Here, it is because there is not enough fuel entering the first cylinder bank. Although this will most likely happen when at low speed or on start, it can occur as engine rpm increases because there is not enough air to maintain that speed.

Bad Starting Behavior

A lean running condition can cause your vehicle to stall on idle. It can cause it to idle at the incorrect rpm and a rough idle can occur. These symptoms are common with most air intake issues but can point to a P0171 trouble code.

How to Diagnose and Fix the Cause of P0171 Code

So, you’ve identified the symptoms and causes. Now we will delve into the diagnostic process of this trouble code and finally fix the root cause.

OBD-II Reader

To take the guesswork out of the equation, you need to use an OBD-II scan tool. This will not only help identify the P0171 trouble code but also any other codes that may be related to your issue. Connect your scan tool and scan for all trouble codes.

You can also use the scan tool to actuate your fuel pump.

Visual Inspection

Engine issues will often exhibit some external sign if there is something wrong internally. We can use that to our advantage. If you suspect your MAF, or fuel pressure sensor to be faulty, give them a quick visual inspection. Check the component itself, but also check the wiring harness and the connector for any issues. Fraying, exposed wiring, melting, or corrosion on the connector means that it could be the source of your trouble code.

Check your fuel filter, fuel pump, and fuel lines. If there is any obvious damage that could compromise these items, you can write them down as the potential culprit behind your lean condition. 

A faulty air intake system is also a common cause of this trouble code. Look for any loose-fitting hoses, any obvious crack in the intake past the MAF, a dirty air filter, or a compromised MAF sensor.

Wiring Harness Test

If everything appears to be solid, you can go ahead and test the wiring harness. All you need is a multimeter. Before conducting this test, check your owner’s manual for the exact wire colors and configuration.

We will be testing where the wiring harness meets the MAF sensor connector by back probing the connector itself. Set the multimeter to 20V DC. Take your red multimeter probe and contact the red (positive) wire. Your black probe needs to touch the ground. You can use the battery ground or the chassis of your vehicle for this purpose.

Turn your car on, but do not start it at this stage. If all is good, you should get around 12V from the live wire, 0.5V from the signal wire, and 0V from the ground.

MAF Sensor Test

Now we will move on to testing the MAF sensor itself. Back probe the MAF sensor signal pin with your red probe and ground with the black probe. If you’ve found the constant ground wire on the MAF connector and want to use that as your ground, make sure that your leads are not touching. Your car needs to be on to test it correctly.

On idle, the output should be around 1V. As the RPM increases, it should rise steadily and decrease in the same way.

Vacuum Pressure Test

This test will help find vacuum leaks in your system. Using a vacuum pump is the most conclusive method of testing for air leaks, but if you don’t have one on hand, a spray bottle with some dishwashing liquid will also work in a less scientific but still effective way.

Connect your vacuum pump to one of the intake hoses and start your vehicle. The pressure should be between 14HG and 22HG, depending on the vehicle. In the same way, use your spray bottle on the intake manifold and hoses and look for any obvious air bubbles.

Fuel System Test

Now that you’ve ruled out vacuum leaks and MAF sensor failure, it’s time to move on to the fuel system. Your fuel injecotrs, fuel pressure sensor, fuel pressure regulator, and pump are the main electronic components that we will test in this system.

You need a fuel pressure gauge for this test. Connect it to your fuel system and turn the vehicle on. As the pump begins to work, it should give you between 20-60 PSI depending on your vehicle. Any rapid drops in pressure or no pressure at all, mean that there are problems with your fuel system.

If you don’t have a fuel pressure gauge for testing, your fuel pump should exhibit an audible hum when it turns on.

How to Fix the Cause of P0171 Code

More often than not, you’ll need to replace some parts in order to fix the P0171 code. The key is figuring out what has failed so that you don’t have to spend a lot of money on parts. In most cases, a thorough diagnostic process should reveal exactly where the issue lies, but sometimes you might end up chasing a very low-profile vacuum leak somewhere in the system. 

Does P0171 Clear Itself?

The sensitive nature of the P0171 code means that it is unlikely to clear itself. Even small vacuum leaks and blockages in the fuel lines can cause this trouble to keep popping up. You should rather clear the trouble code using an OBD-II reader to check if your fix is working properly. 

Is It OK to Drive with P0171?

Even if your car can technically move with this code present, you really shouldn’t be driving any car that is running lean. a lean condition can cause detonations, and those are potentially deadly for your pistons. A lean engine generates a lot more heat than usual, which in turn puts additional stress on all internal components. We strongly recommend that you refrain from using your car until this code is fixed. 

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