by Sam Dillinger (comments: 1)

P0300 Multiple Cylinder Misfire

p0300 multiple misfire

Today we’re going to look at one of the most common, and one of the most confounding, Pcodes that can cause your check engine light to come on. P0300 means “Multiple Cylinder Misfire” and the laymen definition isn’t much different. It simply means that the PCM, Powertrain Control Module, has detected a misfire in more than one cylinder. If you're lucky there will be some other P03__ misfire Pcodes, but there may not be. What does this mean, and more importantly, how do you fix it? We’re going to start with some basic information about the Pcode and then get to some of the common causes associated with this condition.

First let’s take a look at what a misfire is and how it comes about. Vehicles have been using something called a “crankshaft position sensor” for years (it goes hand-in-hand with the electronic secondary ignition system. Think spark plugs for a little clarity). The crank sensor, as it’s more commonly called, is used to tell the PCM exactly where the crankshaft is at in it’s rotation cycle. This is used to deliver the spark to the combustion chamber at the precise best time for combustion to take place, and as a byproduct, to keep your engine running. The crank sensor also monitors engine speed. The physical representation of this is the RPM gauge in your instrument cluster. The RPM signal is read directly from your crank sensor and is routed through the PCM. Using this information the PCM can determine the speed of each cylinder (it’s more complicated than this, and uses more input information, but for the sake of simplicity, we’re going to stop with the technical description here). When one of the cylinders doesn’t complete the combustion process correctly, that cylinder will move slower than the others because that piston won’t be propelled downward by the explosion, instead being drug along for the ride by the mechanical movement of the crankshaft. This reduction in cylinder speed is how the PCM decides that a given cylinder is misfiring.

As I mentioned, it’s more complicated than this, but this is a good working description for what we’re trying to accomplish here. So, when one cylinder isn't moving fast enough, the PCM says that cylinder is misfiring. As you might’ve already guessed, if it detects this pattern in more than one cylinder, you’ll end up with the P0300 Multiple Cylinder Misfire. So how does this happen, what causes it, and how do we determine what that cause might be? We need to start with the type of misfire we’re experiencing and work from there. The combustion process requires air, fuel, compression, and a spark to work properly. If any of those elements is missing, or not operating at peak efficiency, a misfire is the end result. Let’s look at each of these elements briefly to get a better understanding of how they could contribute to a misfire.

We’ll start with air. This one can be easy, but it’s deceptive because it can cause a misfire in ways that aren’t readily apparent. First, if your engine isn’t breathing it isn't running. Period. So the likelihood of not having enough air to your engine is pretty hard to achieve. More like impossible really. But what about too much air? This is how you can end up with a misfire because the air and fuel have to be at an exact mix (14.7 parts air to 1 part fuel. This is known as the stoichiometric ratio) in order to function properly.  If the air part of that ratio gets too high, the combustion process won’t work correctly, and cause a misfire. How do you get too much air? A vacuum leak is how. The air coming into your engine is metered by the throttle body in newer cars. In older cars it would be a carburetor, but that’s a different animal entirely. If any air is induced into the air intake behind the throttle body, this will throw off the ratio, and the combustion process, and can cause the misfire. Incidentally, this is also known as a lean fuel condition. Where there is more air than fuel, so the fuel part of the ratio is considered “lean”. Your engine functions like a big vacuum cleaner in the respect that any time it’s running, it draws in air without stopping. So if you get extra air, you’ll usually be able to hear a higher pitched sucking noise that will aid in diagnosis. You’ll also normally end up with a high idle because the PCM will add more fuel to try and keep up with the extra air, which makes the engine run faster. This will also cause a variety of there Pcodes normally. This is the least likely way to have a misfire, but it can absolutely be a cause, so can’t be overlooked.

Next is fuel. The basics of the fuel system are relatively easy. The fuel pump picks up the fuel from the tank and pushes it through the lines to the fuel injectors. NOTE: the camshaft position sensor is what tells the fuel injectors to dispense the pressurized fuel. This process also will pressurize the fuel. A good guideline for fuel pressure is around 50 PSI +/- 5 psi. Any greater variation than 5 PSI will cause a problem. In order to get an accurate reading you’ll need a fuel pressure gauge, the specific procedure for testing the fuel pressure on your engine, as well as the actual specification for the pressure. If you’re not experienced with doing this kind of test, I recommend as strongly as possible that you DON’T do it. If you did it incorrectly it could lead to a very serious fire. Safety is always more important than anything else when working on your own car. 50 is a general number, though there are quite a few engines that use that as the actual specification, never assume that is correct for your engine. It could lead to a faulty diagnosis and thereby replacing parts that aren’t actually failed. If there’s not enough fuel this will cause a misfire for sure because it just won’t have enough gas to burn properly. This also creates a “lean” condition. If you have too much fuel, this will cause a misfire even more readily. This happens because the ratio of air to fuel only allows enough oxygen to burn a certain amount of fuel. After that oxygen runs out, the burning stops, and the extra fuel will splash onto the spark plug fouling it out, and seriously hindering it’s ability to provide the spark needed to cause combustion. Having too much fuel is known as a “rich” condition. A common cause of too much fuel is a failed fuel pressure regulator. A fuel pressure test will help you find a failed fuel pressure regulator pretty readily. There are other ways that the engine can be getting too much fuel such as failed oxygen sensors. Pull out a spark plug and look at the end. If you’ve got too much fuel, it’ll be sooty black and most likely actually be wet with fuel.

Now we’ll look at compression. This one is a bit more advanced. Here’s a snapshot of the combustion process: the intake valve(s) open and the piston moves down drawing in the air. The intake valve closes creating an airtight chamber (the combustion chamber as a whole) and the piston starts to move up, compressing the air. As the piston nears the top of the chamber, the fuel is sprayed directly into the air mixture in the combustion chamber, then at the peak of the compression, the spark is sent through the spark plug igniting the air/fuel mix. This creates a tightly controlled explosion that propels the piston downward. This process is actual way your engine creates the power to transmit to your transmission, and give you movement. The final part of the combustion process is the exhaust valve(s) opening and the piston traveling up again. This pushes out all of the burnt air/fuel gasses. When the combustion process works perfectly, these gasses are inert, meaning they can’t be burnt anymore than what they are, but they still have mass. This concept is important to how an EGR, Exhaust Gas Recirculation, valve functions. We’ll cover this component in a different article. The valves have to open and close at the exact right time to achieve the correct compression ratio. This is a part of the timing system of the engine. There are compression rings on your piston that seal off the bottom of the combustion chamber as well. If the timing is off, making the valves open at the wrong time, the compression rings are cracked or failed, or the edge of one valve is burnt (this happens from poor combustion) this will cause a misfire. It will also cause a significant lack of power as well. Normally there are several other symptoms that come with a lack of compression, including not being able to start if the lack of compression is severe enough. A failed head gasket or cracked cylinder head can also be causes of a lack of compression. In order to diagnose a lack of compression you’ll need a compression gauge and some pretty advanced mechanical repair knowledge. In most cases I recommend leaving this one to the professionals as it can also be dangerous as well.

The last one for today is spark. This can be the easiest one to detect in a lot of cases. A spark tester is readily available and fairly easy to use. Again, I must caution that if you're not familiar with automotive repair, I strongly urge you to seek the help of a professional. An ignition coil delivers roughly 50,000 volts of energy to the tip of the spark plug. You do NOT want to find out what that feels like by accident! A single jolt from the spark system will whack you from your fingertip to your shoulder and will scramble your thoughts for a minute. At least, that’s what someone once told me… Anyway, you don’t want to experience this particular bit of automotive technology first hand. For those of you who are comfortable with it, just attach the spark tester to the spark plug boot or the single ignition coil, make sure to safely ground the tester, by safely I mean AWAY from the fuel rail, remove the fuel pump relay, and then crank the engine and watch for spark. A good spark should be a nice bright blue and should be consistent with the cranking of the engine. Repeat this process for all of the cylinders of the engine. If you’ve got a bad coil or spark plug, this will help you narrow it down when you find the cylinder that isn’t sparking at all, or not like the rest. If you find a cylinder that is not sparking, you'll need to do some electrical testing to determine why the spark is missing. The spark plug could be failed (the spark tester can also test the plugs), the ignition coil responsible for that cylinder could be failed, the wiring from the PCM to the coil could be failed, or the ignition coil driver in the PCM could be failed. This gets into pretty advanced diagnostics, and is not what we’re here for today. A single failed coil or spark plug won’t normally cause a P0300 Pcode to appear. Under the exact right conditions it could because that single cylinder being off enough can throw off another cylinder that would normally be functioning correctly. As I mentioned earlier, the crank sensor is what tells the spark when to fire, if the sensor fails this will be a very common cause for the P0300 to show up. A quick test is to power brake the vehicle. To do this, apply the emergency brake, push the brake pedal down FIRMLY, put the transmission in gear, and hit the gas. This will put a heavy load on the engine and if the crank sensor is failing, it will make the misfire very pronounced. Do NOT do this unless you are in an area that it is ok for the vehicle to move without causing any damage to anything, and more importantly anyone! If you don’t have safe conditions, don’t try it.

These items are the basics for this Pcode. Believe it or not, I kept it as basic as I could, while still giving you useful information. There is more in depth information for every item I listed, as well as more in depth information for how all of these elements combine. For my members, if you need any diagnostic assistance, want some clarification on the things I've listed here, or would like to stop by and say hi, come and see me at

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Comment by blake E cudworth |

Thank you for the very informative article on P0300 codes. My 2003 GMC sierra 1500HD with the 6.0 has this code. I watched a few videos on possible causes and started by replacing the Sparkplugs and wires..ran great for 2 days and now sputtering again with the same P0300 code. Im not sure where to go next. I'm not a mechanic but i can find my way around an engine. I will be looking at some of the things you listed before i take it to a shop. Thank you

Reply by Sam Dillinger

I’m glad we were able to help a little bit. If you don’t find the issue, make sure to come back and Skype with us to help join find a solution. Good luck!

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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