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P0171 Fault Code

You can find information about the P0171 fault code on this page.

It is one of the most popular fault codes that people search for on this website (in the top 50).

We have given it a seriousness of 7/10 with 1 being the least serious and 10 being the worst.

P0171 Short Description

"System Too Lean (Bank 1)"

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Description Icon Long Description

Fault code P0171 means that the engine control module (ECM) has detected that the first bank of the engine is running too lean.

Running lean is a term used to describe an engine where there is too much air or too little fuel at any given time. Engines require specific ratios to run properly, 14.7 parts of air to 1 part of fuel for petrol engines.

If the powertrain control module (PCM) cannot correct this by increasing the amount of fuel within a certain limit, this will trigger the fault code. This can also result in a P0170 code.

Conversely, an engine can be running too rich. This is when there is too much fuel in comparison to air. This is typically indicated by the P0172 error code.

This P0171 error also informs which side of the engine is suffering the issue. In this case, the first bank is at fault. Also referred to as bank “A” and bank “B.” These can be found in your owner’s manual.

There is also fault code P0174, which indicates that the second bank is running too lean. If both sides are too lean, your vehicle can show both these error codes simultaneously.

While nearly every brand is susceptible to this error, Toyota, Volkswagen, and Chevrolet can be some of the most common to suffer.

Warnings Icon Other Signs of This Problem

As with most problems directly involving the combustion process of an engine, code P0171 comes with a host of fairly typical issues that can easily be recognized.

The most obvious of these is that the Check Engine light will illuminate.

Other signs should be fairly obvious as well. These include a loss of power, a rough idle, difficulty starting the vehicle, and even misfiring when driving. Acceleration should also be difficult.

Broken Down Icon What Could Be The Problem?

Because this fault code represents an issue with the air-fuel ratio, it can mean there is an issue in either system.

There are several sensors that specifically check for this type of issue. Any faults or damage to these sensors can easily result in this P0171 error or P0174 being displayed. The mass airflow (MAF), oxygen, and air-fuel ratio sensors should all be checked.

Another issue is that there is unwanted air entering the vacuum which disrupts the ideal ratio. There can be leaks found in multiple places throughout the engine, particularly the positive crankcase ventilation (PCV) system.

If air is not the issue, then it is likely something in the fuel system. Low fuel pressure can come as a result of a faulty fuel pump and can provide too little fuel. Clogged or faulty injectors and filters can also cause this issue.

Fixed Icon Fixes

Possibly the easiest step to take to fix this issue is to check for and repair vacuum leaks.

Another fix is to replace any damaged or faulty sensors. Again, these will be the MAF, oxygen, or air-fuel ratio sensors.

Replacing any faulty components in the fuel line is also a solution. This can result in the need for one or more new fuel injectors, a fuel pump, or a fuel pressure regulator.

(You can view this video on YouTube for a visual explanation of the problem and how to replace a faulty oxygen sensor to fix the P0171 error)

Danger Icon Seriousness

Any issue directly relating to the running of a vehicle’s engine is fairly serious. Error code P0171 sits in the moderate to severe area.

The vehicle will not be able to run properly. While there is no risk of immediate damage to the engine, it can degrade it over a short amount of time if ignored.

Various other parts have been designed to run at a specific air-fuel ratio and they typically will suffer as a result of this ratio being off.

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