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Check Engine Light System Too Lean - Code P0171 or P0174 on Your Car or Truck

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Created on: 2020-05-28

This video shows you how to diagnose and resolve a problem causing the check engine code P0171 or P0174.

  1. step 1 :What does code P0171 mean?
    • "Code P0171 - system too lean bank 1" means there is too much oxygen or too little fuel in the exhaust, which is referred to as a "lean" condition, or a lean or "low fuel trim" code. This can be caused by something like a disconnected hose or vacuum leak, which means there's more air than usual, or because your fuel system is weak. P0171 is a common trouble code in many vehicles.
  2. step 2 :What is a cylinder bank on an engine? How can you tell Bank 1 and Bank 2?
    • A cylinder "bank," is a row of cylinders. Inline engines, only have one bank (cylinders all in one row), where v-shaped engines like V6's and V8's have two banks (cylinders on both sides of the engine). Flat 4 and 6 cylinder engines like a Subaru might have also have two banks. Each cylinder of an engine is numbered (in service manuals) from the front most cylinder to the rear. Bank 1 is the bank where cylinder number 1 is located, and where you'll find the cause of this check engine light code.
  3. step 3 :Can P0171 be caused by the wrong air fuel ratio?
    • For your car to run the way you would expect it to (read: no check engine light and zero signs of starting or running issues), its engine needs a certain amount of air and a certain amount of fuel. If that air-fuel ratio is out of balance, you'll either notice that something is off, or your car's computer will detect it first. What is that correct ratio? To function optimally, an engine needs a mixture of 14.7 parts air to 1 part fuel (14.7:1).
    • With the help of oxygen sensors, your car's engine or powertrain control module (ECM or PCM) is responsible for measuring the air-fuel mixture and adjusts the amount of fuel depending on the amount of air. If there is too much air, the ECM or PCM overcompensates with larger fuel injections, which triggers the P0171 code.
  4. step 4 :Is it okay to drive with P0171? How serious is a P0171 code?
    • A lean fuel trim code like P0171 causes performance problems that can lead to damage, so it's best to avoid driving with it. These problems can include bad fuel economy, poor engine performance, and even internal engine damage.
  5. step 5 :What are common P0171 code symptoms?
    • Your Check Engine light is on
    • Trouble starting your engine
    • Rough running engine
    • Trouble idling or accelerating
    • Misfiring, stalling, or losing power
  6. step 6 :What are the common causes of code P0171?
    • Airbox snorkel: Find the airbox snorkel that connects to the engine. Grab and shake it so you can feel if anything is loose. If the snorkel appears to be okay, you can also check this area for any other disconnections.
    • Underneath the engine cover: Look for anything that looks disconnected for any open ports that seem like they're missing a hose. Also, check for any blocked ports.
    • Whistling noises while your car is running: Start your engine and listen carefully for a whistling or rushing air sound while you stay away from any moving components under the hood. Hear anything? Start looking in the area where you're hearing noises for a leak, open port, or loose hose. The code's cause could be something as small as a pinhole in one of the lines.
    • Mass air flow sensor: Check for any debris. You may even find a leaf that got through and is blocking the mass air flow sensor.
    • Air filter: Look for any holes or debris. If you find any holes, you'll need to replace the air filter. While you're looking in this area, you can also check for any debris that may have gotten in past the air filter and needs to be cleaned out.
    • Gaskets: Check the gaskets in between the plenum and gasket heads for leaks and cracks.
    • Purge valve seal: If your engine is sounding rough, there could be a large EVAP leak causing the P0171 code. You can also check for a bad seal on the purge valve.
    • Fuel rail: Check the fuel rail's pressure. If the pressure isn't high enough, that's a sign your car isn't getting the amount of fuel it needs, which can trigger the P0171 code.
    • Fuel injectors, filters, or pump: Check to see if these parts are plugged up or blocked, which would keep the right amount of fuel from reaching the engine.
    • Bad oxygen sensors: O2 sensors monitor any unburned or fuel that gets into the exhaust. If an oxygen sensor isn't sending the correct signal to the computer, that will cause an issue and can trigger a P0171 code.
  7. step 7 :How much does it cost to fix code p0171?
    • The cost of repairinf p0171 greatly varies based on the root cause of the problem. Assuming you will be repairing it yourself, the repair could be free if your intake hose just became loose, or a few dollars for a new piece of vacuum hose to replace a cracked one. On the other side of things, p0171 could be caused by leaking intake manifold gaskets, clogged fuel injectors, a malfunctioning mass airflow sensor, or maybe even a lazy oxygen sensor. All of these could run you a couple of hundred dollars.Taking the vehicle to a shop to have the world done for you will obviously increase the repair cost a bit. So why not do it yourself?
  8. step 8 :How do I fix code P0171?
    • Once you've diagnosed what's causing your car's P0171 code (see common causes above), you can attempt to fix it yourself by replacing or repairing the damaged item, and then resetting the Check Engine Light.

So, you have a check engine light and you scan it and you find that you have a P0171, or a P0174, which is a low fuel trim. Let's talk about it. So, here's some quick and easy things you can check that could have caused that problem or can potentially fix it. You wanna find under the hood where the airbox is located. Your air filter is in here and that's gonna...that is where the engine gets the air to breathe, you need air and fuel to make the engine run. So, you need to filter that air. So, off that airbox, you're gonna find a snorkel that goes directly into the engine, you wanna grab that, give it a shake, see if anything's loose, like, over here, like, if this is loose, that's not good. Near the air filter, you're gonna have something called a mass airflow sensor. Now, that's gonna tell the computer how much air is getting into the engine. And the computer needs to know so that it can dump more fuel in or less fuel, depending on that.

But if you're getting extra air coming in through here, it's not measuring that fuel going in. So, that's gonna give you a code. So, you could have replaced the air filter or someone could have replaced the air filter for you and just forgot to tighten this up, it happens all the time. Anything loose like that, you just tighten it up, you should be able to clear the code and you'll be on your way. So, on a different vehicle, the airbox is right here where the air filter is, there's the wires. And if you follow the snorkel, there's actually a hose that's supposed to be right there. If that was left off, that's gonna cause that code or even you check and see it's disconnected right there, that needs to be hooked up. Hook that up and clear the code. So, again, this is a different vehicle, airbox right there, and that's air flow sensor right there. And just check the tubes. You know, you also wanna check any of this rubber. There could be a crack in there and that could cause it as well.

Sometimes these have to be replaced, but definitely check the hose underneath here. You may have to lift an engine cover up, take a look, just make sure everything's connected. All right, so now let's get underneath the engine cover. Many vehicles have little covers over the engine, and all you have to do is lift them up, some of them may have a screw in them. So, you wanna look at the hoses that are connected to the intake, anything, and just make sure nothing's disconnected. Everything's attached. You know, if you had some engine work done recently, and there's a hose that's not connected. Now, you wanna check any ports. If any ports seem like they're open, make sure that it's not something that's plugged off, because that's not gonna cause an issue. But something like this, where it's open, should have a hose on it or something or even a plug covering it.

So, if you don't see anything visual, what you can do is start the engine up and stay away from any of the moving components and try to listen for a whistle-type noise or air rushing. And then just try to feel around, see if you can plug something off. If any of the pitches change, if you plug off one of these holes and it changes, then that can help you find the leak. And also any of these lines, if there was a pinhole in the line, you may be able to find it that way as well. Let's take a quick look at the air filter. Your airbox may have clips or screws holding the airbox down. Just take the air filter out and take a look at it. This one's pretty dirty. Now, that's not gonna cause the code. But if you had a hole in here, where some debris can get past the air filter itself, that's important that you change that. And while you have the air filter out, you can see the mass airflow sensor.

Now, I've seen in the past where you'll have a leaf actually get by the air filter, and it's in front of the mass airflow sensor, and that can cause some issues or any type of debris. It could get dirty and that could cause this code. So, not a very common problem but it could potentially be the mass airflow sensor if it's dirty or sending a bad signal to the vehicle's computer and giving you that faulty code. So, those are some of the quick and easy, simple things you can check out. Let's get a little more advanced. So, everything after the mass airflow sensor into the engine block itself has to be sealed and all the hoses connected. Well, you could also have...you have your intake plenum right here. You actually have gaskets in between the plenum and the heads themselves. Now, those gaskets could leak. They could be broken. They could be cracked. And if those leak, you're definitely gonna get a low fuel trim code.

Now, we have a bunch of videos on how to replace intake gaskets. Every vehicle is a little bit different. You can check those out, try to attempt that yourself. If that's a little more advanced, you can take it to a professional. So, you may notice the engine runs very rough, and there could be a large leak. And something, like, if there's a bad seal on the purge valve or something, you can hear that leaking right now, like that. But you may notice the engine stumbling, running really rough. Another thing we can do is take some carburetor cleaner, or throttle body cleaner. And you can take it while the engine's running. And you can actually feel the engine running rough. And as you spray along the areas that you think you might have a leak at the intake, you can notice the engine either run smoother or rougher, or even have someone watching the gauges and watch the gauges fluctuate as you're spraying. And that's a good indication to find where the actual leak is.

So, what we can do is if we take the scan tool or the code reader, and we can find select engine speed, confirm. Now, while you're monitoring the engine speed, if you have the ability to graph the engine speed, so, and then we're going to take some carburetor cleaner. Now, remember, this is flammable, you don't wanna spray this near the alternator or anything electrical. You don't wanna spray it near the coils or anything. But if you spray it just on the intake gasket itself, or where you think you may have a leak, like, if you sprayed it in that area down there, where we have our leak right here as you're monitoring the engine speed. So, we noticed the drop in revolutions per minute, the engine speed, that's where our leak's gonna be. The engine is running rough. Now, you may see it actually go the other way, and you may see the engine speed go up. As long as you see a difference, that's probably where your leak is. Everything we've talked about so far has to do with the engine getting air and everything being sealed up in the intake system.

So, everything after the mass airflow sensor has to be sealed up to the engine. Now, there could be two additional problems that could cause this code. You could have a fuel injector issue, or an O2 sensor issue. So, your fuel injectors are located right here, and they're attached to a fuel rail. And there needs to be a certain amount of pressure in this fuel rail. So, if that pressure is not high enough, you're not gonna get enough fuel and that's your air fuel mixture is not gonna be correct that could cause that code. So, if there's not enough pressure there, or if the fuel injector itself is plugged up, it's not letting enough of that fuel get into the engine, same deal. So, the engine is not seeing enough fuel in that situation and it's trying to add more fuel and it's unable to do so. That's gonna set that code. Related to the fuel pressure being low, you could have a bad fuel pump not putting enough pressure there, or even a fuel filter that's partially plugged up not getting enough fuel there, or the oxygen sensors.

Now, there's four oxygen sensors on this vehicle. There's two in the front, and there's two that are further down the exhaust. The ones in the front are the ones that are...we're concerned with. Those are the ones that are gonna monitor the engine's fuel air ratio. So, any unburned fuel or air gets into the exhaust and that O2 sensor will monitor that and send the signal to the computer and make adjustments. Well, if this O2 sensor isn't sending the correct signal to the computer, that's going to cause an issue. I hope this video helps you out. If you enjoyed it, please subscribe to our channel. Make sure you ring the bell, turn on all notifications so you don't miss any of our videos.

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