Check Engine Light How to Diagnose Random Misfire and Multiple Codes

Hey, everyone, it's Sue from 1A Auto. We have a '15 Jeep Wrangler with a 3.6 in the shop. It has multiple codes with a misfire. Now, this isn't gonna be vehicle-specific. These codes can actually affect your vehicle too. So here we have our codes that we pulled on this Jeep Wrangler. Now, these are not just vehicle-specific. You'll get these codes on any vehicle. And I just wanna break it down real quick and simple and diagnose this as a easy way in your driveway with limited specific tools. So, but let's solve the mystery together about what all of these have in common. So, your first code is always usually your most important code that could actually be triggering all of them. But let's see it says, "O2 reference voltage circuit low." So these are just abbreviations. So I have an O2 reference voltage circuit low. Why is it low? Is there a broken wire? Is the sensor not working properly? We have to figure that one out. We also have a random misfire, which could be caused by that. We also have fuel system lean Bank 1 Sensor 1. Now, this is a V6. So we have downstream fuel trim 1 lean, which contrives with this one. So all these have one thing in common, fuel pressure. So the first thing I'm gonna do is I'm gonna check my fuel pressure. And here are my specs. So another thing you wanna do, obviously, before I check the fuel pressure, I'm going to check for vacuum leaks, see if I can hear anything. I also like to check my fluids. I'm gonna check the oil here, especially where the code has a random misfire. So that tells me that we've been dumping fuel, hopefully, not too much, but fuel has been going into the oil pan because it hasn't been burning. So I like to check my oil. We just did an oil change on this 2100 miles ago. Oh, yeah, look at that? Look at how black that is. So that is really black.

Let me see if it smells like gas. So that's gonna break it down. You don't wanna leave that fuel in there. Yeah, smells like burnt gas. Oil and gas mix, not good. So you don't wanna leave that in there once you fix your problem because you want the main bearings to be obviously lubed properly. So, now, we have to add an oil change on top of everything else. So, underneath is where I got the fuel pressure on this thing. Fuel lines are right here. You can see them right to the passenger side. There's two lines here, one with a white connector and one with a black with a red lock pin. That's the fuel feed to the engine. There is a line up on top. It has a 90-degree bend to it. You know, if you have the right tool to get to it, then go ahead, but it's harder to reach. This is so much easier to get to. I'm gonna get my pressure from here. Got the gauge set up. We're gonna start her up and see what the pressure is. Vehicle running, our fuel gauge hooked up, we're venting the room, we can read our pressure, and it is at almost 60. And the specs is 56 to 60. So right now we're doing good. This looks good. This isn't gonna cause the O2 low voltage code. It's not gonna cause a random misfire code due to lack of fuel pressure. We also have a lean running code, which if the fuel pump wasn't reading pressure, it's gonna do that. But I have good pressure. So, now, we can eliminate this as a problem unless it's an intermittent fuel pump.

So now that I know my fuel pressure is good and I'm looking at my codes I have over there, I'm gonna check with a scanner. So this is why it's kind of borderline with a DIYer. You've gotta have a really good scanner, something that is gonna get you true time, true readings. I like to use the graph scale so I can actually see the digit numbers for the voltage change, the reference signal. But let's just quickly talk about what other things that operate that can cause a random misfire along with running lean fuel mixture trim, crank sensor. Crank position sensor on this, I know this is supposed to be for all vehicles, but if you have a modern vehicle, you have a crank sensor. Well, that crank sensor is what sends a signal to tell the engine how to fire all the coils. It's part of the system. If you have an intermittent crank sensor, it's not gonna send a signal. And that's why you're chasing the misfire. If you have a constant misfire, like on cylinder two, five, six, whatever, it's a constant misfire, then is more than likely gonna be a cylinder problem, or just a spark plug, or a coil problem, or wire if it's an older vehicle, even an injector problem. But it's specifically that cylinder where we have a random misfire, and I wish we could see live data misfiring, but we're not capable with that with the scanner we have, and actually, this vehicle might not allow you to. But when you get constant or random, and it's intermittent, and right now this thing runs great, it happened on the highway when she accelerated hard to pass. These are kind of known for that. The crank position sensor has a little hiccup in it. We didn't get a crank sensor code, though. So, as a technician, you're gonna start banging your head against the wall because you didn't get a crank sensor code. Just could have been a blip that should have sent the code though. So, now, I have to go deeper into the first code, which is O2 voltage. So if that voltage spiked, sent the wrong signal, that's gonna send wrong information again, and the car might end up with a misfire. O2 sensors will cause a misfire, but we don't wanna just start throwing O2 sensors into this vehicle and then not fixing it and thinking, "Wait, it wasn't an O2 sensor," because that is the biggest mistake a lot of people make, just throw a part in it. We have to make sure that the information going to that part is working properly.

All right, so I customized the screen. So we can actually see real-time voltage for our O2 sensors and long-term and short-term fuel. So here we go with...this is Bank 2 Sensor 1. Up sensor, that's beautiful. I like that a lot. Bank 1 Sensor 1, not so much. Not doing what I think it should do. Bank 2 Sensor 2 is actually going and changing way too fast. That should be a lot slower. Bank 1 Sensor 2, look at the difference. So these are both Bank 1 and Bank 2 downstreams. Look how this one's reading. Now, we also have a code for this. So at this point, I see one or two sensors that I'm happy with. Short fuel trim is gonna be faster. This one's not looking too good. This one's normal. And long-term is good. So to do this right, I think I'm gonna start with checking for vacuum leaks, which I already know I don't have. I know my fuel's pressure is good, still holding good, held pressure when I shut it off for over a half-hour. It's not really misfiring at all right now, but if I was to take it out on a load and drive it down the road, I could start getting a misfire due to the readings that this is gonna give.

So before we just replace O2 sensors because of what we saw on a reading, I always do a visual because you wanna make sure that the harness to the vehicle prior to the O2 sensor is not damaged because it's gonna give a bad reading for that sensor. You don't wanna get the sensor then say, "Oh, I'm ready to put it in," and go to get to that point, and you're like, Oh, the harness before, it's no good." So, I like to do a quick visual, make sure I don't see anything else going on here, and I actually already did it, and I found a problem. Kind of common, oil leaks, Jeeps. The valve cover on this vehicle is leaking. And, look, the sensor that's giving us a faulty code is covered in oil. So there it is. So, now, instead of getting four O2 sensors because I wasn't happy with all of them, but, you know, they're gonna cause each other to have bad reading sometimes. That O2 sensor right there is definitely no good. So I have to do a valve cover gasket and a new O2 sensor, clear the code, and let's run this thing. Don't forget, if you know that your vehicle has been running lean or rich... I already checked the dipstick, and the oil on this smells like fuel. So that's gonna cause a random misfire due to the fact that it doesn't know what to give for fuel, too much fuel or too little. So, do an oil change also. So that pretty much wraps it up for the code 1621. We have the O2 sensor with oil all over it. So it shorted it out, bad reference voltage, and that's what we did, and it fixed it. But I did a little bit more homework for you real quick, just so you know, if you check your O2 sensors out, there's no oil on them, no broken wires, they're in good shape, and you do a run-out test and a graph, and you've got voltage going, there is an update to be flashed from your local dealer for that code, so just means that it just needs to be reprogrammed. Try that out first before you go throwing parts at your car. So, if you're not a subscriber, please subscribe, and don't forget to ring that bell because then it turns on all your notifications, and you won't miss a future video.