P0171 is a very common code for any ODB2 vehicle. What does this code mean anyway? The definition for this this P0171 code is Fuel Trim System Lean (Bank 1). I have have been dealing with this P0171 diagnostic trouble code for many years on many different year make and model vehicles. Everyone always asks me is “what is the fix for this P0171 lean code?” Everyone thinks there is one fix for all. I hate to break it to you, but there is no “one” fix for this P0171 code. This code just simply means the engine is running “lean”. Not enough fuel for a given amount of air or too much air for a given amount of fuel. As soon as the computer sees the air fuel ratio go too lean for a period of time two consecutive engine warm up cycles, it will flag your P0171 code. Well, let me ask you, what can cause an engine to run lean? This is the question I have to ask myself every time I diagnose one of these codes. If you have an air or a vacuum leak, would the engine run lean? Of course it would! The computer will see that the air fuel ratio is off and will light the check engine light and store the P0171 lean code that can be retrieved using a generic code reader or a scanner. If you have low fuel pressure, would the engine run lean? You bet it would! What if the engine had plugged fuel injectors or if one of the sensors on your engine was bad not allowing enough fuel into the engine while it was running? You guessed it! It would run lean and will definitely cause your P0171 diagnostic trouble code. If you have a lean code and would like some help, click here to chat with a mechanic online.
Diagnosing P0171 trouble code
Now, your next question should be, “how do I diagnose a P0171 lean code?” This my friend, is the 64 million dollar question. Just like a doctor to a sick patient, every case is different. Depending on what type of vehicle you are working with will depend on what could be wrong which will therefore dictate how you are going to go about diagnosing this P0171 code. The cars computer determines injector pulse width based on the inputs it receives from various sensors. Based on this, the engine will run rich or lean. Ideally, you want it to run right in the middle. If one of these sensors are off or “skewed”, the computer will run the engine either slightly rich or lean depending on how the sensor goes bad which will flag a P0172 Fuel Trim System Rich code (Bank 1). But again, this is just one side of it. Even if all of the sensors under the hood are good, if you have an air or a vacuum leak or if the fuel pressure is too low, then it will cause the engine to run lean and cause your p0171 lean code.
Checking for air and vacuum leaks
The first step in diagnosing a p0171 lean code is to check for air or vacuum leaks. This is a very important step! I cannot even begin to tell you how many air/vacuum leaks I have come across resulting in a p0171/p0174 lean code and or causing a major idle or performance problem! Most major vacuum leaks can be found by just by opening the hood and listening for a hissing sound while the engine is at idle as it will be very obvious. However, not all vacuum leaks are easy to find. Sometimes the intake manifold gasket, throttle body gasket or even a vacuum line or hose will leak engine vacuum which will result in a P0171 code in the computer. Sometimes these leaks are hard to find. If you suspect a small vacuum leak causing your p0171 lean code, you can use a smoke machine to smoke check the intake manifold to help find the leak. A smoke machine is actually made to smoke the Evaporative Emissions system for leaks. However it works great for finding an air or a vacuum leak. The smoke pro makes a pretty good product. It is a bit expensive, but i will save you a tone of time. In addition, if you get one, you can make a couple of bucks smoke checking your friends and families Evaporative Emissions and intake systems when the time comes.
Checking fuel pressure for P0171
If there are no air or vacuum leaks, you are going to want to rule out low fuel pressure. Low fuel pressure can certainly cause the engine to run lean which will obviously flag a P0171 lean code. However, I do not see this one very often causing this code. Don’t get me wrong, low fuel pressure is very common, but the customer will come in with a low power or even an no start complaint before the computer gets a chance to set a p0171 lean code in the computer because of low fuel pressure. But if you think you have low fuel pressure, you can check out this link to read about how to check fuel pressure.
Another thing that can cause your p0171 lean code would be incorrect valve timing. It sounds strange, but if the valve timing has jumped, then the intake valve or valves for each cylinder will not stay open as long as they should while the piston is moving down. This will prevent the cylinder from getting all of the air in needs for the combustion process and will result is a lean condition. Engines with timing chains will usually not jump time. Import vehicles (including some American vehicles) with timing belts are much more likely to jump time for various reasons which can possible trick the computer and cause the P0171 lean code. I am not going to get into this too much here because it does not happen very often but I just wanted to bring it up so that you are are of this. If you think this may be an issue with your vehicle, you can check the engine vacuum at idle to see if it is nice and steady. The engine vacuum will vary depending on the vehicle you are working with. However, just about all four stroke internal combustion engines (that are street legal) will produce anywhere from 18 to 22 inches of vacuum at idle. It is very important that the engine vacuum remain steady at idle. If it is bouncing around or going up and down slightly as the engine is idling, then you could have a mechanical issue or even an air or a vacuum leak like we talked about earlier. If the valve timing has jumped, then the engine vacuum will be a bit lower than normal at idle. You can pick up a Vacuum Gauge at Amazon.com for only 20 bucks.
You see, there can be a few things that can cause this p0171 lean code. We did not cover everything, but at least you get the idea of the lean condition and what can cause this code. I hoped you learned from this article as I tried to keep it very basic. We will cover more about this p0171 lean code and other possible causes at a later date. 🙂