A P0171 code is set by your vehicle’s powertrain control module (PCM) when it detects that the air-to-fuel ratio of your vehicle is too lean. This means that the PCM is relaying that the engine is receiving too much air into the combustion chamber and not enough fuel.
The PCM receives signals from the oxygen sensors located in your vehicle’s engine bay and is constantly working to adjust the right amount of air and fuel based on the readings provided by these sensors.
When the factory predefined range falls out of line, more specifically, is lower in fuel than the motor needs, the PCM will trigger the P0171 code.
The PCM then calls for more fuel to be sent through the fuel injectors and into the combustion chamber of the engine. This action is called a fuel trim, and in the case of a P0171 code, where more fuel is called for, this is a positive fuel trim.
What Does the P0171 Code Mean?
The Code P0171 stands for “Fuel Trim System Too Lean (Bank 1)”.
When the ECM/PCM is having to add too much fuel based on input from the O2 sensor, the fault code P0171 will be sent to the ECM, which will trigger the check engine light to illuminate.
This code could either mean that the oxygen sensor is detecting too much oxygen or too little fuel in the mixture.
This lack of fuel is known as a lean mixture. In simpler terms, what the code P0171 means is that your system has too lean of a mixture and is not operating optimally.
A vehicle running on a lean mixture causes more pollution because it emits oxides of nitrogen (NOx) that are harmful to the environment. Lean vehicles also have a tendency to misfire as hydrocarbons from unburnt fuel reach the catalytic converters.
This could cause a spike in the car’s fuel consumption, as well as exhaust damage and emission test failure.
If you wish to learn more about rich and lean OBD codes, you can read our discussion about air-fuel mixture. To gain a technical understanding of how the air-fuel mixture can trigger codes to be stored in a vehicle, you can read our explanation about the ideal mixture and OBD codes.
Otherwise, continue reading the following section to quickly learn the possible causes of P0171.
When the P0171 code sets, the oxygen sensors detect too little oxygen in the exhaust (running “lean”) and the control module adds more fuel than normal to sustain the proper air/fuel mixture.
- Rich Mixture = Too much fuel, not enough air
- Lean Mixture = Too much air, not enough fuel
Symptoms of this DTC
When your vehicle has a P0171 code, there are several symptoms you might notice. These include:
- Engine stalling or rough idling
- Reduced power and poor acceleration
- Limp mode
- Difficulty starting the vehicle
- Check engine light illuminated
- Unusual engine noises, such as knocking or pinging
- Overheating engine
Common causes of this DTC
Several common issues can trigger the P0171 code, including:
- Dirty or faulty mass airflow (MAF) sensor
- Vacuum leak in the intake manifold, hoses, or Positive Crankcase Ventilation (PCV) system
- Faulty oxygen sensor
- Clogged fuel injectors
- Dirty fuel filter
- Weak fuel pump or low fuel pressure
- Faulty fuel pressure regulator
- Exhaust leak before the O2 sensor
How to Diagnose Code P0171?
It’s vital to perform an OBD-II scan of your vehicle before assuming the issue’s cause. You or a mechanic should scan for Code P0171 to ensure this is the correct diagnosis for your automobile and that no other fault codes are showing up upon seeing a check engine light.
You might think that changing out some sensors will resolve the issue, but a proper OBD-II scan will confirm the problem takes place around Bank 1. It is recommended that someone with experience in the diagnostics field check for Code P0171.
A professional mechanic should have some additional tools to get to the bottom of the issue.
Below is a list of areas to inspect after finding Code P0171. Complete these steps in the order until you uncover the reason for the error message:
- Use a vacuum gauge to locate leaks —only when the engine is off.
- Check fuel sensor and mass flow air sensor performance with a fuel pressure gauge.
- Follow manufacturer instructions for testing oxygen sensors throughout the fuel system.
- Once you’ve tried all other options, the problem likely lies in the power control module.
How Do I Fix the P0171 Code?
You can resolve the Bank 1 system running too lean error using a few approaches. Depending on the issue’s cause, you may be able to turn the check engine light off by completing one or more of these repairs:
- Replace the fuel pump or fuel filter
- Change the fuel pressure regulator
- Replace the powertrain control module
- Change one or more injectors
- Replace one or more oxygen sensors
- Change the mass airflow sensor
- Repair vacuum leak
To prevent the P0171 code from returning, you can maintain your vehicle regularly, such as with oil changes and tune-ups.
Why Does a P0171 Code and Running “Too Lean” Matter?
“Lean” running cars and light trucks are highly polluting vehicles. Most of the NOx pollution, which is poisonous and can cause asthma, is caused by vehicles that are running too lean.
A lean running car can also misfire, which puts raw fuel (HCs) into the catalytic converter which can result in internal damage, and on into the atmosphere. When you are behind a car or truck that is misfiring it makes your eyes burn.
In comparison, a “rich” running engine (one that is not misfiring as a result) has no smell (CO is odorless) or you may detect a rotten egg smell, which is sulfur dioxide produced by the Catalytic Converter.
P0171 is not an Oxygen Sensor problem. Before a P0171 code is possible, the computer first ran a series of tests to validate the readings from the oxygen sensors.
Since the oxygen sensors passed their readiness tests and didn’t set any codes, the computer then looked to the Fuel Trim adjustment. When the computer determined the air-to-fuel mixture to be too lean, it then set the P0171 code.
What if There Is No Vacuum Leak and Codes P0171 is Set?
An “underreporting” Mass Air Flow Sensor can be a common cause of a code P0171. Essentially, this means that the Air Flow Sensor is telling the computer that much less air is entering the engine than actually is.
Since the oxygen sensors are telling the computer that more fuel is needed, this causes confusion in the computer because the Mass Air Flow Sensor is still saying there is too little air and the Oxygen Sensor is reporting that the mixture is still too lean.
The computer tried to compensate, but since resolution is impossible, it sets the code. It is important to restate that the Oxygen Sensors are accurate—the fuel mixture is too lean.
In this case, the Air Flow Meter or Sensor is inaccurately reporting the real amount of air entering the engine.