Car or Truck with Low Power or Hesitation Code P0171 Low Fuel Trim Diagnostic

Check engine lights and fuel trim issues with a code P0171 are some of the hardest to diagnose. I'm going to show you a cool new trick in this video.

Here we have a first-generation Toyota Matrix with a check engine light. No obvious drivability issues, but we scanned it, and we found that we had a fuel trim lean code. The reason why we get this low fuel trim code has to do with the amount of air getting into the engine. The computer monitors the airflow going into the engine. How it does that is through the mass airflow sensor located right here.

We have the airbox here with an air filter. So filtered air goes past the mass airflow sensor, air goes in here passed the throttle body into the intake and into the engine. Now everything from the intake up here, up to the mass airflow sensor has to be sealed. If any air gets in any of these gaps or any of these hoses, you're going to get a code.

The first thing I want to do is check the air filter and make sure there's nothing wrong with that, make sure it's not dirty or even missing. So the air filter looks good. It's a little bit dirty, but not too bad. Next, we could look at the mass airflow sensor itself and make sure there's nothing stuck to it, make sure it doesn't look dirty, make sure there's not a leaf stuck to it because that can cause an error itself. Now let's check the snorkel. Just grab a hold of here, if any of these are loose, then those would need to be tightened down. And that all looks good. And all these vacuum lines look good as well.

A very common problem on these Toyota Matrixes is the intake itself and the gasket. The intake could warp and cause the gasket not to seal properly. And that could be our issue. So one way to test this is to run the engine. As it's running, spray some carburetor cleaner, all along the areas of where the intake meets the head. And if you see the engine stumbling, or if it changes the way the engine runs, most likely you have an intake leak and the gasket's gonna need to be replaced, or possibly the intake itself. But we're gonna test this a little bit different.

What we're gonna do is take the snorkel off and the throttle body. The reason I take the throttle body off is because it's electronically controlled, and I don't want to mess any of the electronics up. And we're going to hook up a Shop-Vac backwards. So it's going to be blowing or compressing air into the intake and we're gonna spray it down with soapy water, and wherever we find the bubbles, that's where it's leaking from.

All right. Let's turn it on, see what happens. Use soapy water. Look at the bubbles right there.

So if you look at this gasket, it is completely crushed. And that's not sealing anything. So the only thing that was working here was the intake itself right up against the head. This is the way it's supposed to be, the gasket is raised up a little bit. And when you compress the intake against the head, that compresses the gasket. So then it gives you a good seal. We didn't have a good seal, so, that's why we got the code.

Now, if you're doing this job, you potentially could just replace the gasket and see if that worked for you. The intake itself could have worked a little bit and you may be in the same situation. So, in some cases, replacing the intake is the better way to go.

So that's a different way to diagnose a fuel trim issue. If you enjoyed this video and it helped you out, make sure you subscribe to our channel, ring the bell, turn on all notifications so you don't miss any of our videos.