If your car has a check engine light on for a P0300 code, it means a misfire has been detected. Your car may also be running rough and you also might be experiencing bad gas mileage.
In this article and video, we review how to diagnose and fix the ignition coil that may have triggered the P0300 code, which stands for “Random/Multiple Cylinder Misfire Detected.”
Check Engine Light? Misfire Detected – Code P0300 – Ignition Coil Diagnosis
What Is Code P0300 and What Does It Mean?
Code P0300 means your car detected a random or multiple misfire. On an OBD2 scan tool, you’ll probably find this code defined as “Random/Multiple Cylinder Misfire Detected.” You might also have a reading for other P0300 codes, like from P0301 to P0308. These codes have a number at the end that refer to a specific cylinder.
Here are some 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
What Is a Misfire?
A process called “combustion” occurs inside internal combustion engines (ICE). These are very precisely timed small explosions that happen multiple times per second.
This combustion is fueled by your car’s gasoline, and it’s ignited by the spark plugs. If any part of the combustion process isn’t working perfectly, the engine “misfires,” which means the combustion isn’t happening as your car’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? Can I Drive with Code P0300?
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, like when driving up a hill or accelerating
- Bad gas mileage
More about common misfire symptoms
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
How Do I Diagnose and Fix a P0300 Code?
Use a Scan Tool or Code Reader to Determine Which Cylinder Is Misfiring
With the engine running, use an advanced 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. Check to for signs of defectiveness like melting or damage.
If it doesn’t look like there’s anything wrong with the ignition coil connector, next, remove that cylinder’s ignition coil. 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 ignition coil to test whether the misfire moves to that other cylinder.
After you’ve moved the ignition coil to a different cylinder, if the misfire moves to the with it to that cylinder, you’ll know that specific ignition coil is causing the misfire.
If One Ignition Coil Is Bad, Should You Replace all Coils at the Same Time?
Ignition coils are a wear item. When one ignition coil fails and needs replacing, the others probably aren’t far behind if they’ve been working for some time. It’s perfectly safe to replace just one if needed, but many technicians will recommend replacing them all at once, along with your spark plugs, if they’re due to fail. 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 P0300 Video: