P0300 code: Mechanic advice to diagnose and fix

Our mechanic shows you how to diagnose and fix the problem causing a P0300 code (“Random/Multiple Cylinder Misfire Detected”), and uses an OBD2 scan tool, like this one, to help.

So, your car has a P0300 code, or P0301 code or 302, etc., —your Check Engine light is on, your car is likely running rough, maybe even sputtering, or you’re experiencing bad gas mileage. 

In this video, our mechanic shows you how to diagnose and fix the ignition coil that may have triggered the P0300 code (“Random/Multiple Cylinder Misfire Detected”). Watch now:

Check Engine Light? Misfire Detected – Code P0300 – Ignition Coil Diagnosis

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What is code P0300 and what does it mean?

P0300-P0309 are some of the most common engine misfire-related codes.

  • P0300 – Random or Multiple Misfire Detected
  • P0301 – Cylinder 1 Misfire Detected
  • P0302 – Cylinder 2 Misfire Detected
  • P0303 – Cylinder 3 Misfire Detected
  • P0304 – Cylinder 4 Misfire Detected
  • P0305 – Cylinder 5 Misfire Detected
  • P0306 – Cylinder 6 Misfire Detected
  • P0307 – Cylinder 7 Misfire Detected
  • P0308 – Cylinder 8 Misfire Detected

As our mechanic explains in the video above, a P0300 code means a misfire has been detected. On an OBD2 scan tool, you’ll probably find this code defined as “Random/Multiple Cylinder Misfire Detected.”

What is a misfire?

Inside every engine, there are very precisely timed small explosions that happen multiple times per second, which is a process called “combustion.”

This combustion is fueled by your car’s gasoline and ignited by its spark plugs. If any part of the combustion process isn’t working perfectly, the engine “misfires,” which means the combustion isn’t happening as the engine’s computer expected. It is the engine version of an unexpected cough.

Specific P0300 codes, such as a P0303, indicate which cylinder the misfire is happening in, so a P0303 code means a misfire is detected in cylinder 3.

The vehicle our mechanic is working on in the video has codes P0300, P0303, and P0306, as well as primary/secondary circuit codes, which may help narrow down what could be causing the misfire (or multiple misfires) in cylinders 3 and 6.

How serious is a P0300 code?: Is it OK to drive with P0300?

You should not drive with a P0300 code because a misfire should be addressed immediately to avoid catalytic converter and internal engine component damage.

What are common P0300 code symptoms?

P0300 error code symptoms can include: 

  • Your Check Engine light is on or flashing, which means your catalytic converter could be at risk of being damaged.
  • Your engine is running rough or sputtering, especially when under a load (up a hill or accelerating.)
  • Bad gas mileage

Learn about other common misfire symptoms.

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What causes a P0300 code?

P0300 causes can include: 

  • Ignition issues, including faulty, damaged, or failing ignition coils
  • Faulty or failing spark plugs or wires
  • Fuel injector issues: they could be plugged or stuck open
  • Faulty ignition coil circuits or other ignition coil circuit or wiring issues
  • Mechanical engine issues, like damaged or improperly functioning valves
  • Old or contaminated fuel

Learn about other misfire causes.

How do I diagnose and fix a code P0300?

Use an OBD2 scan tool or code reader to determine which cylinder is misfiring

With the engine running, use a scan tool or code reader to watch the cylinder misfire data. You’ll be able to see how many misfires are occurring in each cylinder. 

Once you know which cylinder you need to check, locate that cylinder.

How to determine which cylinder is number 1:

Looking externally at your engine, anywhere where there is a spark plug, there is a cylinder right below it.

On a v-shaped engine, the front-most cylinder (closest to the rubber belts), is cylinder number 1. Odd-numbered cylinders (1, 3, 5, 7) are on one side of the engine, and the even-numbered cylinders (2, 4, 6, 8), are on the other side. Flat 4 and flat 6 cylinder engines are numbered similarly to v-shaped engines.

If your vehicle has an inline 4, 5, or 6 cylinder engine with all of the cylinders in a row, the cylinders are numerically in order from front to back. The front-most cylinder (closest to the rubber belts) will be number 1. The rear-most cylinder (closest to the transmission) will be the highest number (4, 5, or 6).

You can also find your car’s cylinder layout by looking up “Firing Order” in your owner’s manual.

Inspect the misfiring cylinder and coinciding ignition coil

With the engine off, disconnect the ignition coil connector of the misfiring cylinder and check to make sure it’s not melted, or that there isn’t anything else that could be preventing it from working properly (at 2:00 in the video).

If it doesn’t look like there’s anything wrong with the ignition coil connector, next, remove that cylinder’s ignition coil using a 10-millimeter socket (at 2:12). Check the coil for anything noticeably wrong, such as any melting. If it doesn’t look like there’s anything wrong with the coil, swap it with another coil to test whether the misfire moves to that other cylinder (at 2:30).

If the misfire does move to the cylinder where that ignition coil was moved, you’ll know that ignition coil is causing the misfire.

If one ignition coil is bad, should you replace all coils at the same time?

When one ignition coil fails and needs to be replaced, the others are probably not far behind. It is perfectly safe to replace just one if needed, but many technicians will recommend replacing them all at once, along with your spark plugs. That way your entire ignition system is fresh, and you won’t need to worry about more vehicle downtime when the next ignition coil begins to fail.

Tools used in our featured P0300 video:

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