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

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.