P0300 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

P0300  Random/Multiple Cylinder Misfire Detected

Meaning

A quick refresher regarding how the spark plugs and the cylinders work together to help your car run smoothly.

Most cars will come with between four and six cylinders. The more cylinders the engine has, the more power it generates. If you want to go fast, you need more, not less. 

However, if you want to save on fuel costs, fewer cylinders will make your vehicle more economical.

Each cylinder has a spark plug connected to it. As the name suggests, the plug fires a spark, which ignites the fuel in the cylinder. This process happens rapidly, and each cylinder fires in sequence, over and over again.

So, where does all this energy go? The burned fuel powers the crankshaft, which is what turns the wheels of your car. When you press on the gas, you burn more fuel, which moves the crankshaft faster, increasing your overall speed.

When a cylinder misfires, this process gets interrupted. If there are multiple misfires, it could affect the speed of the crankshaft, which is measured in rotations per minute (RPM). The impact of misfiring can either increase or decrease the rpm, which can damage the engine if it happens enough.

The P0300 code will activate if the differential of these misfirings is more than two percent. If the impact is between two and ten percent, the check engine light will come on. Hopefully, the misfirings won’t affect the rpm more than 10 percent, as that is a serious problem. In that case, the check engine light will flash on and off to warn you to get it fixed ASAP.

So, if your light is on and you see the P0300 code, you know that the misfirings are happening in more than one cylinder, and they are affecting the rpm of the engine by more than two percent. It’s imperative that you correct this issue immediately so that it doesn’t worsen over time and damage the engine block or the crankshaft.

Causes

Unfortunately, there are many different reasons why the cylinders may be misfiring, which means that pinpointing the issue can take a while. Let’s go over the most common causes of this problem. We’ll get into how to identify and fix them later on.

  • Worn or Damaged Spark Plugs
  • Worn or Damaged Spark Plug Wires (or Coils)
  • Worn or Damaged Distributor Cap - this piece distributes the voltage to the spark plugs
  • Worn or Damaged Rotor Button - this component manages the electricity that flows through the distributor cap
  • Defective Fuel Injectors
  • Clogged Exhaust Gas Recirculation (EGR) Valves - these valves recirculate exhaust to help control the temperature in the engine
  • Incorrect Ignition Timing - the timing of when the sparks ignite
  • Vacuum Leaks - air is getting into the cylinders
  • Low Fuel Pressure
  • Leaking Head Gaskets - this piece sits between the cylinder heads and the engine block
  • Cracked Distributor Cap
  • Faulty Camshaft Sensor
  • Faulty Crankshaft Sensor
  • Faulty Mass Air Flow Sensor
  • Faulty Oxygen Sensor
  • Faulty Throttle Position Sensor
  • Faulty Catalytic Converter - this component traps toxic gases before they flow out through the exhaust
  • Faulty Power-Train Control Module (PCM) - the unit that manages each part of your power train

Symptoms

Unless you’re checking your OBD2 codes regularly, it can be tricky to determine if the problem is a misfiring cylinder or something else. Here are some warning signs that could point to this particular issue.

  • Flashing Check Engine Light
  • Car Doesn’t Start or Takes Longer to Start
  • Vehicle Dies When Stopped
  • Rough Idling (excessive vibrations or noise)
  • Hesitation When Accelerating
  • Less Power When Driving
  • Increased Fuel Consumption

How to Diagnose the Cause of a Misfiring Cylinder

Because there are so many potential problems that can cause your cylinders to misfire, it’s imperative that you go through each one step by step. Never assume anything either, so don’t skip over any steps because you don’t think it could be a specific item. For example, if your car is relatively new, you may assume that the spark plugs should be in pristine condition, so you don’t check them. However, faulty spark plugs can happen, so be sure that you’re thorough in your inspection.

Step One: Use Your OBD2 Scanner

Retrieve all of the freeze frame data and the trouble codes that are stored in your vehicle’s primary control module (PCM). Be sure to clear them all so that the check engine light comes off.

Step Two: Drive the Car

In some cases, the issue may be a faulty sensor, so all you have to do is reset the code. However, if the light returns and you see the P0300 code again, you will have to do a full inspection of the engine to determine the cause.

Step Three: Check OBD2 Scanner for Live Data

Hopefully, your scanner can provide this information. If not, you may want to borrow one that can. Check the data on your cylinders while the vehicle is running to see which ones are showing misfires. Remember, this code means that multiple cylinders are misfiring, so let it run for a while until you identify them all.

Step Four: Inspect Your Spark Plugs and Coil Pack

When checking the plugs, be sure to look at both the wiring and the plug itself. You’re looking for any signs of wear and tear, as well as any significant damage such as cracks or ripped wires.

The coil pack is a device that houses ignition coils to control how the spark plugs fire into the cylinders. Inspect the pack for any signs of damage, including cracks, breaks, or corrosion.

If you notice any damage, be sure to replace any of these components. Once you’re finished, make sure that you reconnect everything correctly. If necessary, consult a manual to ensure that you don’t make a mistake.

Step Five: Reset the Code and Run the Car

Hopefully, replacing the spark plugs and/or the coil pack will solve the problem. Make sure that you reset the check engine light and clear all problem codes so that they won’t interfere with your progress.

If the spark plugs, wires, and coil pack weren’t the issue, now it’s time to move to the fuel injectors.

Step Six: Inspect the Fuel Injection System

Modern cars use computers to manage the fuel injection system, so you may not be able to inspect it without taking your vehicle to a mechanic. However, if you have an older car, you should be able to look at them for signs of damage or wear.

Step Seven: Inspect the Distributor Cap and Rotor Button

As with the fuel injectors, these components are not accessible on newer models. You will have to take your car to a professional to see if they are damaged or need replacement.

Step Eight: Check Your Car’s Compressor

Realistically, you should never have to get this far to identify the cause of your cylinder misfires, but in rare cases, a faulty compressor can create this issue.

Step Nine: Check Your PCM

If you’ve inspected everything, it all works well, and the P0300 code still returns, it may be an issue with the PCM itself. Faulty sensors can trigger warnings even when everything is working, so you may have to replace the module itself. Again, this is rare, so you shouldn’t have to get this far. Replacing your PCM should only be a last resort.

Common Mistakes

As we mentioned, never assume anything. Usually, people will make the mistake of thinking that it can’t be something specific (i.e., spark plugs), either because the car is still new or the components were recently replaced.

Another common mistake is not resetting the trouble codes on your OBD2 scanner so that you can be sure you’re retesting the engine and not seeing an old code.

Finally, don’t rule out anything we mentioned on the diagnostic guide. Although items like the PCM and compressor are rare, they can affect how cylinders and spark plugs interact. Be as thorough as possible in your inspection.

How Serious is a P0300 Code?

The question you’ll probably be asking is, “can I drive with misfiring cylinders?” Realistically speaking, when the check engine light comes on, you can drive for a little while, but address the issue as quickly as possible.

Misfiring cylinders can get worse over time, and it can damage your engine. Also, you will lose fuel efficiency, so plan on paying more for gas until you get it fixed.

Overall, P0300 is a serious issue, and if you notice any significant problems while driving (rough idling, slow acceleration), you need to take care of it immediately.

What Repairs Can Fix P0300?

We talked a bit about what you can do to correct this issue, but let’s run down all of the various repairs that may be necessary to ensure you don’t continue to have cylinders misfiring.

  • Replace the Spark Plugs
  • Replace Spark Plug Wires or Coil Pack
  • Repair or Replace Clogged EGR Valves
  • Fix any Vacuum Leaks
  • Repair or Replace Damaged Head Gaskets
  • Replace a Faulty:
  • Camshaft Sensor
  • Crankshaft Sensor
  • Oxygen Sensor
  • Throttle Position Sensor
  • Fuel Injector
  • Catalytic Converter
  • PCM

Conclusion

Realistically, the reason your car’s engine is misfiring is because of an issue with the spark plugs or wires. Unless you have an older car that hasn’t been well maintained, fuel injectors or faulty sensors shouldn’t be the issue.

Overall, make sure that you consult a mechanic so that you can be sure to correct any potential problems. As we’ve mentioned, misfiring cylinders can have lasting damage on the engine and your fuel system, so don’t wait to fix it.