If you have a vehicle with a P0171 error code, the onboard computer (ECM) is reading a problem with the air/fuel ratio.
An OBDII scan tool will give the error message “P0171 “Bank 1 System Too Lean.”
The P0171 code is a notorious engine error that can be caused by several things.
Unlike many ODBII codes, which point to one specific problem or component, the P0171 code is more involved.
While it is more difficult to troubleshoot, following some basic steps and ruling out possible bad components will help to narrow down and solve the problem.
The procedures to solve a P0171 code will be the same across all vehicle manufacturers, including Ford, Dodge, Chevy, Toyota, Nissan, Kia, and more.
What Is a P0171 OBDII Error Code?
A P0171 code indicates there is a fuel/air mixture problem on Bank-1 of the engine.
Bank-1 is the side of the engine with the number one cylinder.
A 4-cylinder will only have 1-Bank, while a V6 or V8 engine will have 2-Banks.
Find the side of the engine with the number-1 spark plug, and that will be the side having the issue.
The term Lean means the cylinder is receiving too much oxygen or too little fuel creating a bad lean air/fuel mixture.
The air/fuel mixture needs to be correct when it goes into a cylinder, or it will not burn correctly.
Which Failed Components can Cause a P0171 Error Code?
- Failing Mass Air Flow Sensor
- Clogged Fuel Filter
- Bad Fuel Pressure Regulator
- A Weak or Failing Fuel Pump
- Vacuum Leak
- Bad Oxygen Sensor
- Bad Injector
Possible P0171 OBDII Code Vehicle Symptoms
- Engine Light Stays On
- Loss of Power
- Engine Hesitation
- Engine Difficult to Start
- Engine Dies Easily
- Black Smoke From The Tailpipe
Possible P0171 Error Code Fixes
- Replace or Clean Mass Air Flow Sensor
- Replace Fuel Filter
- Replace Fuel Pump
- Replace Fuel Pressure Regulator
- Find and Fix any Vacuum Leak
- Replace Injector
- Replace Oxygen Sensors
Troubleshooting and Diagnosing a P0171 OBDII Error Code
- Check Fuel Pressure
- Mass Airflow Sensor
- Check for a Vacuum Leak
- Test or Replace the Fuel Injectors
- Bad Oxygen Sensor
Testing the fuel PSI with a fuel pressure gauge will quickly show if the problem is the fuel or airflow.
A fuel pressure gauge is attached to a valve on the fuel rail and will give a pressure that should match the engine’s rated PSI.
For example, a Chevy 5.3L engine is rated for 55-62 PSI at the fuel rail. If the pressure is lower, then low fuel pressure is the issue.
If the PSI is normal, then the problem is most likely an airflow problem.
A weak or failing fuel pump can cause the pressure to be off, messing up the mixture.
A clogged-up fuel filter or pressure regulator can also cause this problem.
Several components must work together to get the right air and fuel pressure mix, which includes the mass airflow sensor (MAF).
A vehicle computer uses the mass airflow sensor to adjust the air mix into the engine.
For this reason, the mass airflow sensor is commonly cleaned or replaced to fix a P0171 error code.
A vacuum leak can cause the mass airflow sensor to get a bad reading and cause a P0171 code.
There are several methods to test for a vacuum leak, including a pressure test or smoke machine test.
Some mechanics will spray a liquid around a possible leak location to see if the engine idle changes.
A smoke machine is also often used with smoke fed into the system, which will come out at the leak location.
Once a vacuum leak or mass airflow sensor is ruled out next step is to test or replace the fuel injectors.
The fuel injector may be failing, causing a bad mixture to go into the cylinder.
There are different methods to test the injectors to see if one has failed.
For example, If a cylinder is also giving a misfire code, often the injector can be swapped out with a known good to see if the problem follows.
The onboard computer (ECM) receives data on how much oxygen had been burnt off in the cylinder after combustion using an oxygen sensor.
If an oxygen sensor fails, the data to the onboard computer will be off, which will cause an issue.
Modern cars and trucks often have two oxygen sensors or more called upstream and downstream 02 sensors.
Different methods can be done to troubleshoot a bad 02 sensor.
For example, an OBDII scan tool’s live data stream feature can help greatly to see if an oxygen sensor is having an issue.
Have you had a vehicle with a P0171 OBDII error code? Let us know your thoughts below.
I have a 2013 frs and out of no where it turned off on me on the highway. a month or 2 it started to start 2-3
seconds after ignition but it was rare. A week before my car turned off, there was a slight knocking/tapping noise after 80mph that sounded like it was infront of the driver side. I had it scanned and the only thing that came up was the P0171 code. I changed the tank fuel pump thinking it gave out and it did not work. I took out the hpfp (did not take out the part holding/under the hpfp) and the coil looks good, so did the roller so i didnt think much of it but Im thinking it could be it considering the slight knocking noise at a higher rpm/mph, it being heard infront of the driver side and not knowing if the problem could be inside the hpfp. I checked the ohms for both o2 sensors and marked good, aswell as cleaned the MAF sensor and it wasnt that. but I tried checking the fuel pressure on the lines but it is so difficult to install, and also the fittings i rented are not capable for the 3 fuel lines on top of the injectors. I did however check the middle fuel line which i think is the out and it had 0 psi. Tomorrow im going to check the fuel injections to see their ohms and how well they spit out fuel. I would be more than blessed to have your opinion to help with this 1 month problem, thank you
A P0171 code can be hard to troubleshoot, even for seasoned mechanics, since it can be caused by many different things. If the 02 sensor is good, you could pull the spark plugs one at a time and match them to a spark plug burn chart to see if it is an individual cylinder causing the issue or the whole bank. If it is the whole bank, I would test the fuel pressure with a gauge.