Engine Misfire Trouble Code P0303 Meaning Diagnose Spark Plugs Ignition Coils

Hi, everyone. Sue here, 1A Auto, and I want to talk to you about code, PO303, and that is a engine misfire in cylinder 3. Now, this is gonna be for any vehicle that has a V6 in it, or even a 4-cylinder, even a V8. I'm just gonna show you how to do a quick diagnostic. You can grab a little scan tool if you have one, at least to pull the code, or you can go to your local garage, have them pull the code for you. And if you want to look, check it out yourself and not pay those fees, here's some easy steps to follow.

So on this particular V6, we have...firing order is, one, two, three, in the back. So this is cylinder three, right here, and the car does have a rough running, and it shakes quite a bit, and the code is, PO303, so that means cylinder three directly. So what I'm...first thing I'm gonna do is I'm gonna take off the ignition coil. And that's a direct coil firing ignition. So we have...this is what they look like, if you can't see it in the back. Well, you might have plug wires, and you might have just a 4-cylinder with direct coil, or cap and rotor. First thing we're gonna do, is go to that cylinder and see what we find. So on this model, it has an Allen head bolt style that holds it down in. I'm just gonna loosen it up. I'm gonna take that mounting bolt off, and I'm gonna pull this ignition coil right out, so we can get to the spark plug.

As you can see, this is the wiring harness. There's a clip here, just pull on that clip, and lift up. So once you have pulled the coil, you can give it a good check over, examine it. If you see cracks in the plastic here, down in here, this is the boot that covers the electrode that goes inside there. If you find traces of black soot or white, that means that this coil has cracked in its internally. So when it's firing, it's actually grounding out through the engine block. But, I don't see anything too obvious, visually, on this ignition coil. So next step to do is, let's pull the spark plug. So on this car, we have a small type spark plug, it's just a 5/8 socket. With an extension, I'm going to bring it down in there, bottom it out, and I'll just take my ratchet, break that free. Make sure you don't drop it.

So, you see all the black soot, and it looks wet, right? It's got a gloss to it. So that tells me that this spark plug is not firing. It's trying because it's got a few little spots right here, but it's really full of black soot and it's not looking too good. I'm going to do something real simple if you don't have a compression scanner, some tools like that. So I'm going to swap, I'm going to take coil from the front and a plug from the front, and I'm going to swap them. I'm going to run this and double check. Clear the codes first, if you can. Clear the codes, then run it, and let's see if the skip switches over to a different cylinder. And that'll tell us, it's either gonna be the coil, or the spark plug. And if it's either one, I'm gonna do both at the same time. So now that I took cylinder three apart, spark plug and coil, I'm going to swap it with cylinder five, up here on the front, right in the middle here. So I'm going to transfer the spark plug and this ignition coil over to cylinder three.

We can see the spark plug's nice and clean. It's not misfiring. So let's see what happens after we swap it. So now I'm going to take my scanner and I'm going to clear that code that has PO303 on it. Because I took a good known cylinder, spark plug and ignition coil, and I swapped it over, back and forth. So then, I'm going to run it and the engine's still gonna shake because hopefully, it's a bad coil, or spark plug, which is an easy fix. But we'll still have a misfire, but hopefully it will swap right over to cylinder four. Let's give it a shot.

So now that you see that we swapped the cylinders, three and five, and cleared the code prior, started it up and ran it, and yep, the misfire came right out of cylinder five. So you change all the coils instead of just one because if not, you're going to be chasing that around. And you want to do the job once, do it right. And that way, you don't have to deal with it, and you can spend your time with your family and friends. So don't forget, if you liked this video, hit subscribe, ring that bell, turn on all your notifications. I'll see you next time.