One of the most common trouble codes with the Toyota 4Runner is the P0171 code. This code number references a number that is given to you when you plug your car into a OBDII scanner. It’s not the most complicated OBDII trouble code to diagnose. It is also one of the most common.
P0171: Fuel Trim System Lean Bank 1
4Runner 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 vehicle’s 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.
4Runner P0171 Code Symptoms
The P0171 code on a Toyota 4Runner 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 4Runner to misfire.
- Idle Issues– With the air/fuel mixture altered, the vehicle can idle erratically as the computer compensates to keep the engine running.
This youtube video is a great resource for diagnosing the P0171 trouble code in your 4Runner.
Top Toyota 4Runner P0171 Causes
There are many things that can cause the P0171 trouble code on the Toyota 4Runner. Here are the most common ones.
- Vacuum Leaks– With the Toyota 4Runner, one of 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.
- Oxygen Sensor– The P0171 codes is thrown when the oxygen sensor(s) do not register enough oxygen in the exhaust. When the oxygen sensor is bad, it can cause a false reading and trigger the code. The computer will still adjust the air/fuel mixture to compensate. This will cause the 4Runner to run rich.
- 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.
- Air Filter– While it’s not the most common reason to throw the P0171 code, a clogged air filter certainly can cause the code to throw. And, since it’s so easy to check, why not look?
Good luck figuring out what is wrong with your 4Runner. If there are any inaccuracies, or anything that you feel could improve the article, please feel free to comment or message. Thank you.