P0171 GMC Sierra

One of the most common trouble codes with the GMC Sierra is the P0171 code. This code number references a number that is given to you when you plug your truck into a OBDII scanner. The code is often accompanied by the P0174 code, which is for bank 2.

P0171: Fuel Trim System Lean Bank 1


GMC Sierra P0171 Code

P0171 and P0174 GMC Sierra–  If you have both of these codes at the same time, this is where you should start.

Sierra P0171 Code Defined

P0171 is an OBDII trouble code. The code technically stands for:

Fuel Trim System Lean Bank 1

The code is thrown when the oxygen sensors detect that there’s not enough oxygen in the exhaust system. This is commonly referred to as a “lean” condition.

The vehicles ECM will add more fuel to the mixture in order to bring the air fuel mixture where it needs to be for optimal combustion. It also throws the code P0171 to let you know that it has done this.

Sierra P0171 Code Symptoms

The P0171 code on a GMC Sierra will often be thrown if the fuel injection system is not operating properly. Typically the code will be triggered by a lean condition in the engine.

There aren’t usually any drivability issues associated with P0171. For most people the first sign that anything is wrong at all is the service engine soon light coming on.

Here are the typical symptoms that something is wrong when you have code P0171.

  • Mileage- may or may not suffer depending on how much the air fuel mixture has changed.
  • Misfire– If the air fuel mixture has gotten too far from factory specs, this can cause the Sierra to misfire.
  • Idle Issues– With the air/fuel mixture altered, the vehicle can idle erratically as the computer compensates to keep the engine running.


GMC Sierra P0171 Causes

P0171 Sierra

There are many things that can cause the P0171 trouble code on the GMC Sierra. The most common problem is far and away the first one listed below.

  • Vacuum Leaks– With the GMC Sierra, the most common cause for the P0171 code to show is a vacuum leak. Take a look around and see if you can find the vacuum leak. Here’s a great YouTube video on how to find a vacuum leak.
  • Mass Air Flow Sensor– The mass air flow sensor calculates the volume of air entering the engine. If it has gone bad, the air fuel mixture will be off and the P0171 code will show.
  • Bad Fuel Injectors– If you have one or more fuel injectors that have gone bad that can certainly cause the air/fuel mixture to go bad.
  • Fuel Pump/Fuel Filter– Today’s modern fuel injected motors need a lot of fuel pressure to atomize the gas properly. If the fuel pressure is not as high as it needs to be this atomization doesn’t happen properly and the optimal air/fuel mixture isn’t achieved.


Good luck figuring out what is wrong with your Sierra. If there are any inaccuracies, or anything that you feel could improve the article, please feel free to comment or message. Thank you.

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