What Does the OBD-ii Trouble code P0300 Mean?
P0300 Random, Multiple Misfire Detected means that your car’s computer (PCM) detected that there is a misfire in random or multiple cylinders. Code P0300 is often related to bad spark plugs or spark plug wires but could be caused by a number of issues relating to ignition and the air-to-fuel ratio in the cylinders.
When the air/fuel mixture in your cylinder(s) is not able to burn normally and the difference is severe enough, it may cause a misfiring cylinder problem in your vehicle. It’s a serious problem that needs to be fixed immediately.
Your vehicle moves when gasoline is burned and power is generated inside a chamber known as the cylinder. Most engines have a 4, 6, or 8-cylinder engine, where more cylinders typically mean more power. Power is generated by pistons that move up and down at very specific times as fuel is ignited. A cylinder misfire typically occurs when the timing of this ignition is off. P0300 indicates that two or more cylinders are experiencing misfires.
You can take your car into a shop for diagnosis, or click here to save money by fixing P0300 yourself with FIXD!
Can I Drive With P0300 Code?
No, you need to stop driving as soon as you see P0300. Depending on whether you want to fix it yourself or how far from home you are, you might want to get your car towed to a mechanic. It is dangerous to drive with this trouble code because your car may quit in the middle of the highway.
How Serious Are P0300 Codes?
Our advice: P0300 (when misfiring occurs in random or multiple cylinders) is bad. A misfire is a very serious problem for an engine to have. If you see this fault code you should stop driving immediately, figure out what is causing the trouble code, and fix it. Driving with P0300 can destroy your engine, which can cost over $4000 to replace.
Driving with P0300 can also damage other parts of your car. P0300 can be caused by an excess or lack of fuel in your air/fuel mixture. Both of these conditions can damage your catalytic converter. Replacing your catalytic converter can cost $100s-$1000s.
How Hard Is It To Fix P0300?: A few reasons for a “misfire detected”
Yes, you may be able to fix P0300 yourself. Depending on the cause of the trouble code, the diagnosis and the repair might be simple or might be very difficult to do at home even if you have all the right information and advice. The repair might be as simple as replacing a spark plug but, the repair might be more complicated too. You might need a new timing belt or head gasket, and not everyone can or wants do that.
If a “misfire detected” warning like P0300 comes up, the safest thing to do is get your car towed to a mechanic. If you continue to drive with P0300 you may destroy your engine.
Can I Fix P0300 Code Myself? (DIY to Pro Difficulty Table)
Yes, you may be able to fix P0300 yourself. Depending on the cause of the OBD code, the diagnosis and the repair might be simple or might be very difficult to do at home even with all the right information. The simplest repairs can be done with common tools like a ratchet and screwdriver, but the more complicated repairs require specialty tools.
| Cause | Repair | DIY Or Mechanic | Resources |
|---|---|---|---|
| Spark Plug Worn Or Broken (most common cause of P0300) | Find the spark plugs that are worn and replace them | This is a simple repair that you can do yourself | How To Replace Spark Plugs How To Read Spark Plugs |
| Loose Or Damaged Coil Pack Connectors | Tighten or replace the loose or damaged wires | This is a simple repair that you can do yourself | How To Secure Loose Or Broken Coil Pack Connectors |
| Worn Spark Plug Wires | Find the spark plug wires that are damaged and replace them | This is a simple repair that you can do yourself | How To Change Spark Plug Wires |
| Worn Or Broken Ignition Coils | Find the worn or broken coil and replace it. In some cars it can be difficult to access the coils. | This may be something you want to try at home. If you have trouble accessing the coils go to a mechanic. | How To Replace Ignition Coils |
| Engine Timing Is Off | You may need a new timing belt. This can be a very tricky repair and doing it incorrectly can damage your engine. | If you have a lot of experience working on your car you can try this yourself. Otherwise, it’s best to take it to a mechanic. | How To Replace Your Timing Belt |
| O2 Sensor Worn Or Broken | Your O2 sensors/wiring might be worn and need to be replaced | This is a simple repair that you can do yourself | How To Replace O2 Sensors |
| Damaged/Dirty Fuel Injector | Your fuel injectors need to be replaced or cleaned. | This might be challenging depending on your experience, but you can try this at home | How To Clean Fuel Injectors How To Replace Fuel Injectors |
| Vacuum Leak | You need to find where the leak is happening and close it | This might be challenging depending on your experience, but you can try this at home | How To Find And Fix Vacuum Leaks |
| Low Fuel Pressure | This might be caused by a bad fuel pump or fuel line breaks. You Need to find what caused it and fix it. | This might be challenging depending on your experience, but you can try this at home | How To Tell If You Have Low Fuel Pressure |
| Leaky Gasket Head | The repair might be as easy as a using a sealant on the gasket head or as difficult as a full replacement | Depending on the severity you can apply a head gasket sealant yourself. If the leak is more severe and the gasket is blown the repair can take over 10 hours and is very difficult | How To Use Head Gasket Sealant How To Replace A Head Gasket |
How Do I Fix a P0300 Code? (Engine Misfire)
You can fix P0300 at home. Before you fix P0300 though, make sure you correctly diagnose the cause of the OBD code first. If you don’t, you could end up spending $1000s on replacing parts that weren’t the cause of the problem. Here’s a few tips on where to look, and a little advice on what to look for.
Worn Or Broken Spark Plugs
Worn or broken spark plugs are the most common cause of p0300. If you find the spark plugs are worn or broken you will need to replace them. Make sure you inspect the spark plugs to see if there is an excess of fuel or oxygen in your fuel mixture. If the spark plug leads are black there is likely too much fuel in the mixture. If the spark plug leads are white, there is likely too much oxygen in the mixture.
Loose Coil Pack Connectors
Check for loose connectors or damaged wiring at the ignition coils. Look for loose engine ground wires as well. These can cause random misfire conditions. Tighten or connect where necessary
Damaged Spark Plug Wires
If your spark plug wires are worn, frayed, or broken, you will need to replace them. Replacing spark plug wires should be part of your regular maintenance. They should be replaced about every 50,000 miles. Over time, their electrical resistance gets too high and can cause misfires.
Damaged Spark Plug Coils
If any of your spark plug coils are worn or broken you will need to replace them. Spark plug coils are responsible for sending an electrical pulse to the spark plugs that trigger the spark in the cylinder. If these are not working you will have misfires.
Engine Timing
If your engine timing is off, you might need to replace your timing belt. The timing belt is one of the most important parts of your engine and keeps your engine in sync with itself. If it’s off by even a little bit you can have severe problems. You should regularly replace your timing belt every 100,000 miles. This is a complicated repair and if it’s done incorrectly it can destroy your engine. Unless you are sure you can do this yourself, you should take it to a mechanic.
Broken O2 Sensor
If your O2 sensor is worn or broken you will have to replace it. You’ll want to check the wiring too. The O2 sensor lets your car adjust the amount of fuel it adds to the air/fuel mixture. If the O2 sensor isn’t sending proper readings to the computer (PCM) then it could be sending too much or too little fuel to the cylinder. This can cause misfires.
Broken/Dirty Fuel Injectors
If any of your fuel injectors are broken or dirty you will not have enough fuel in your fuel mixture. You will have to either clean or replace the fuel injector. A broken or clogged fuel injector can result in a P0171 code too. If the fuel injectors are leaking you may have too much fuel in your fuel mixture and may have a P0172 code.
Vacuum Leak
If you have a vacuum leak you need to find where the leak is and fix it. Your air intake system might have a leak in it so it could be taking in more air than it is measuring. Since the car only injects the right amount of fuel for the amount of air it detects, this can lead to a lean fuel mixture. You may see a P0171 code.
Low Fuel Pressure
If you have low fuel pressure you will need to find the cause and fix it. It might be a broken fuel pump or leaking fuel line. This may cause your air/fuel mixture to not have enough fuel int it and you may see a P0171 code
Leaky/Blown Head Gasket
If you have a leaky head gasket you may have to seal or replace the head gasket. Sealing the head gasket is a simple repair. You can buy a head gasket sealer that does the trick. If the leak is severe though you may have to replace the head gasket entirely. This is a very complicated job. Unless you have a lot of experience and are okay with long repairs you may want to take your car to a mechanic to get this repair done.
Once you’ve fixed the problem make sure to clear the P0300 code and drive for about 15 minutes to see if the OBD code returns. If it returns, you have not fixed the problem.
You can clear the P0300 code with the FIXD sensor.
What Causes P0300 Codes?
The most common reason for code P0300 is worn or damaged spark plugs. There are other potential causes for this OBD code though. It’s important to look for other codes that could explain why the converter failed.
An incorrect air to fuel ratio could also cause misfires. Lean means there is not enough fuel in the ratio and rich means there is too much fuel. Look for the following OBD codes to see if your engine is running rich or lean.
A faulty catalytic converter can also cause misfires. If the exhaust is not able to get through the catalytic converter properly, exhaust can build up in the cylinders throwing the air/fuel mixture out of balance. You may see code P0420 if a failed catalytic converter is causing misfires.
The most likely cause of a P0300 code depends on your car’s model and age. Use FIXD’s tool to see what the most likely cause of this error code is for your specific car (Also search our site for more information & advice on your car or anything car related).
Other possible causes are:
- Faulty or worn spark plugs and/or spark plug wires (Read our detailed guide on how to replace worn spark plugs at home)
- Ignition issues, including failing or damaged ignition coils
- Distributor failure
- Faulty fuel injector
- Dirty Air Filter
- Low Engine Oil
- Vacuum leak
- Low fuel pressure
- Camshaft and/or crankshaft sensor defective
- Engine timing off
- Faulty / Leaking head gasket
- Low engine compression
- Poor quality fuel that is old or contaminated
How To Diagnose a P0300 Code (Cylinder Misfire)
There are five major things to check when diagnosing P0300:
- Make sure there are no other codes present (address those codes first)
- Make sure coil pack connectors are tight
- Check the condition of the spark plugs
- Check the fuel pressure
- Check engine compression
Look for the following codes to see if you can find a misfire detected on any specific cylinders. These codes can help you pinpoint which cylinder is misfiring
P0301 corresponds to the first cylinder, P0302 corresponds to the second cylinder, and so on. Look at your owner’s manual to identify how the cylinders are labeled.
Make sure it’s the only code showing for your check engine light
If there are other OBD codes you need to address them first because they could be the cause of the p0300 code. You can use an OBD2 scanner like FIXD to see if there are other codes present. If there are other codes present like P0420, P0171, or P0172 they could be the source of the problem. Faulty catalytic converters or other faulty parts in the emissions system are often able to affect each other and can lead to a tricky diagnosis, on the bright-side, solving them will help you learn.
If you have the OBD codes P0171 or P0172, you have too much or too little fuel in your air/fuel mixture and you should address this first since this might be the cause of your P0300 code.
You should be able to find the source of a multiple cylinder misfire by looking at the all the codes that your engine is throwing. Look for the following codes to see if you can find which cylinder(s) had a cylinder misfire. These codes can help you pinpoint which cylinder is misfiring
P0301 corresponds to the first cylinder, P0302 corresponds to the second cylinder, and so on. Look at your owner’s manual for more information about how the cylinders are labeled.
Check The Coil Pack Connectors
This is the simplest solution. A loose connection can lead to too much resistance in a spark plug ignition coil, which can prevent the fuel from igniting in the cylinder. Look for loose engine ground wires as well. These can cause random misfire conditions. Tighten or connect where necessary.
Check The Condition Of The Spark Plugs
Worn and old spark plugs or spark plug wires are common causes of random misfires or, if a couple plugs or wires are bad, possibly multiple cylinder misfires. Replace spark plugs and wires if needed and recheck for misfires.
> How to replace spark plugs in 4 easy steps
> How to identify a fouled spark plug
> How to test spark plug wires
Check To Ensure Your Engine Is Getting The Proper Amount Of Fuel
If you have determined that your ignition system is operating correctly, there may be a problem within your fuel system that is causing the random misfires. The following should be checked to ensure the engine is getting the proper amount of fuel.
Check fuel pressure: Low fuel pressure can cause intermittent misfires on multiple cylinders. When the pressure is below the specification, the engine does not receive the proper amount of fuel and will start to lean misfire. The fuel pump or fuel pressure regulator could be the source of the low fuel pressure.
Check fuel injectors: Make sure the fuel injectors are functioning properly and activating. Random misfires can be a sign of faulty or clogged fuel injectors that need to be replaced. Also, check that the fuel injector wiring is not damaged and is connected properly.
> How to check fuel injectors by ear
> How to check fuel injectors with a digital multimeter
Perform Engine Compression And Leakdown Test
If the ignition system and fuel system checks out, you may want to perform an engine compression test and leakdown test to see if there are any mechanical problems causing your misfire.
Common mechanical problems that can cause misfires:
- Leaking Head Gasket
- Broken valve spring
- Broken piston ring
- Worn valve guides
- Burned valve
- Timing chain or belt skipped tooth and engine is off time
What Are The Symptoms Of a P0300 Code?
When you have a P0300 code you may notice the following:
- Check Engine Light is on or flashing
- Engine runs rough, hesitates, or jerks when accelerating
- Drivers may not notice any adverse conditions when driving
- In some cases, drivers may experience decreased fuel economy, fuel smell from exhaust, rough idling, or lack of power from the engine
- Commonly associated with OBD codes: P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308
How to Fix
With a misfire fault, the first step is to get it diagnosed to figure out what is causing the engine to misfire. We’ve created this 100% free step-by-step guide to show you how to diagnose and fix your P0300 code.
Click here to download your FREE P0300 Fix-It Guide!
If your vehicle is misfiring and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal certified shop nearby to pinpoint the problem and give an accurate estimate for repairs.
These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing. We also recommend using our OBD2 Code Cost Estimator to get an idea of how much it will cost to fix the issue. This tool can help you make an informed decision and avoid overpaying for repairs.
How Much Does It Cost to Have a P0300 Code fixed?
Once properly diagnosed, P0300 may require one or more of the following repairs to resolve the underlying issue. These prices are based on national averages and include parts and labor. Your cost may differ depending on your location and type of vehicle.
Ignition Coils: $230-$640 (some cars require Intake manifold removal)
Spark plug wires: $180-$240
Fuel injectors: $1500-$1900
Vacuum leak: $100-$200
Fuel pump: $1300-$1700
Fuel pressure regulator: $200-$400
Fixed It But The Check Engine Light Is Still On?
Check engine light sometimes need to be reset manually, check out our article:
How to Reset Your Check Engine Light | 4 Ways To Clear It (With or Without a Scanner)
At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.
