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P0171 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


System Too Lean Bank 1


Over time, everything falls apart, and automobiles are no exception to this rule. O2 sensors wear out, hoses crack as they age, and filters fill up and have to be replaced Your vehicle’s computer can compensate by extending the timing for each fuel injector for a fuel rich mixture, but eventually this measure is going to fail, too. 

When the inevitable happens, your vehicle is going to store code P0171 – System Too Lean Bank 1. A P0171 can often pop up with a P0174 (System Too Lean Bank 2). Ideally, the stoichiometric ratio for your fuel system is 14.7 parts air to one part fuel, which provides a balanced  mixture of fuel economy, performance and lower emissions during each combustion cycle.


There are many interdependent systems within your vehicle that can affect the fuel-air mixture. The troubleshooting process isn’t exhaustive, but it does require you check specific causes like the ones listed below.

  • Dirty Mass Airflow Sensor (detects the amount of airflow coming in via the mass airflow post-primary air filter.
  • Vacuum Leak (airflow hoses are cracked, broken, or not properly sealed)
  • Bad Oxygen Sensor(s) (these wear out all the time, and depending on the type of fuel you run and your vehicle may need to be replaced more frequently)
  • Low Fuel Pressure (could indicate a problem with the fuel pump or fuel injectors)
  • Running Out of Gas (it is always important to verify that there is fuel in the vehicle and that your fuel gauge is working properly)


The first thing you’re going to notice is your vehicle runs/idles rough when sitting or accelerating. High idle, engine misfire, power loss when accelerating, or even engine stalling can occur before you ever see a Check Engine Light on your dashboard. Typically, you will experience these symptoms at lower speeds and RPMs as the engine can cope with lean air or fuel flow at high speeds thanks to the increased air pressure. If you start to notice any of the above problems, take your vehicle in for service ASAP.


A competent technician will conduct a thorough diagnostic to determine the root cause of a P0171 or P0174 code. It is essential to follow a step-by-step procedure so that no probable cause is missed during service. Here’s a breakdown of effective troubleshooting steps for a P0171/P0174 trouble code:

Check all vacuum hoses for leaks, including seals, lines, and hoses to ensure they are properly attached and connected. You can either listen for hissing sounds created by escaping air while the engine is running, or use a vacuum gauge if you have access to one.

Next, check the vehicle fuel pressure.  Using your OBD-II scanner, you should get a reading result within your car manufacturer’s specifications (usually available online or in maintenance manuals). Once you’ve verified fuel pressure and checked the vacuum air flow system, verify the mass air flow sensor readings if code P0171 isn’t resolved. Usually, you just need to clean the mass air flow sensor, but if a cleaning doesn’t work it’s best to inspect the oxygen sensors as well.

Lastly, if nothing else has resolved the problem, test your  powertrain control module (PCM) using your OBD-II scanner. Follow these steps exactly, as doing so provides you with a better understanding of your car’s status and the cause of the problem registering the trouble code.

Common mistakes

Do not skip steps in the troubleshooting process.  Many DIYers swap out one or both O2 sensors as soon as they see the trouble code. Here’s the problem with skipping right to the O2 sensor exchange: if the root cause is a dirty or faulty mass airflow sensor or vacuum leak, the O2 or A/F sensor will read differently to compensate. Don’t replace parts unnecessarily. Read and analyze those fuel trims and all freeze-frame data: that’s the  key to properly diagnosing a P0171.

How serious is this?

Aside from running/idling rough at lower RPMs and slower speeds, you may start stalling out at traffic lights or intersections when your vehicle is at rest. It may take a while to reach that point, but it can be a serious inconvenience as it begins to happen more frequently. You’ll also notice that you are probably spending more at the gas pump because you have to “goose” the accelerator to keep your engine from stalling out.

What repairs can fix the code?

Closely following an effective diagnosis procedure is critical to resolving this trouble code. Depending on the problem and how long you let it go, the repair could be cheap and fairly minor to replacing multiple expensive parts. You may only need to replace a broken vacuum hose or air seal that’s letting in surplus air the computer cannot account for.

Additionally, a clogged fuel filter will restrict the supply of fuel, and that’s a minor repair. That being said, finding and fixing both problems can be a time-consuming process. Plan to drop your vehicle off all day so your mechanic can find the problem and recommend repairs.


What does it mean when bank 1 system too lean?

P0171 is the shorter-term code for the system is too lean in bank 1. In other words, this code indicates the first engine bank is not functioning correctly, resulting in fuel not being delivered to the engine in the correct amount.

Is it safe to drive with the P0171 code?

If your vehicle has the P0171 code, you can drive it for a short period of time. A prolonged period of driving with this code can cause the engine to overheat and damage the internals.

How can you tell if your mass airflow sensor is bad?

  • During acceleration, there is a hesitation, stalling, or jerking.
  • There is an excessive amount of air to fuel.
  • The air fuel mixture ratio is too lean.

Related codes

None listed.

P0171 Code – What Does It Mean & How To Fix It