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The Complete Guide To Diagnose And Fix P0300 Yourself

Watch the video below then follow the steps to diagnose and fix your P0300 code! Don’t have FIXD yet? Click here to save 67% for a limited time.

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With the FIXD Sensor and app, you can view your car's live OBD2 data to pinpoint issues, see the most likely repair for P0300, get expert help from our Mechanic Hotline, and more. Click below to save 67% now.


How To Find The Cause Of Your P0300 Code

While check engine lights are never fun, P0300 is the most common diagnostic trouble code drivers will experience. The cost to fix this code at a repair shop can range from $250-$1900 on average, depending on the issue. But don’t worry, we’re breaking down how to diagnose and fix this code yourself so you can save money.

Before you can fix P0300, you need to identify the root cause of the misfire. The most common cause of a P0300 code is worn or damaged spark plugs, but this varies by make and model. Since there are many other issues that could also trigger this code, we’re going to walk you through each possible cause step by step. Click on sections below to learn more about each possible cause.

Want to see the most likely repair for your car’s P0300 code, get detailed diagnostics and live OBD2 data, and more? Try FIXD today for just $19.99!

Are there any other error codes?

Difficulty Level: Beginner

Tools Needed: OBD2 Scanner

P0300 may not be your only code. Before you fix anything related to P0300, make sure you don’t have any other error codes present because they will need to be addressed first. For example, a p0171 (faulty oxygen sensor) can also cause misfires. If you replace your oxygen sensor without fixing the root cause, your new oxygen sensor may fail, too.

The only way to know if you have other codes is to use an OBD2 scanner like FIXD.

Unlike other cheap car scanners, FIXD isn’t a bulky handheld device that just spits out a code you have to Google for actual information you can use.

With the pocket-sized FIXD Sensor and app, you’ll be able to instantly translate your check engine light into simple terms on your phone and clear it with the touch of a button. FIXD tells you what your engine code means, how serious it is, and potential fixes with diagnostic steps. You’ll also be able to read live data from your car’s sensors to diagnose your problems. With FIXD Premium, you can even call the FIXD Mechanic Hotline to get specific answers for your car’s problems. Click here to get FIXD for just $19.99 for a limited time!

Are your fuel trim readings normal?

Difficulty Level: Beginner (with FIXD)

Tools Needed: OBD2 scanner with live data

An engine runs best when it has just the right mixture of air and fuel inside the
cylinder. At idle, this ratio is typically 14.7:1 in a gasoline engine, or 14.7 parts air to one
part fuel. If there’s too much fuel, the spark won’t burn it all. If there’s too little fuel, the combustion process won’t produce as much power. Either of these can cause a misfire condition.

To avoid this, the engine management computer constantly checks and adjusts the amount of fuel going into each cylinder to keep this mixture as close to 14.7:1 as possible. These adjustments are known as the fuel trim.

Use FIXD to measure fuel trim:

You can determine whether your misfire (p0300) is an ignition problem or a fuel problem by measuring the fuel trim on your vehicle.

This is something that a mechanic can do for you, or you can save money by doing this easy test yourself with an OBD2 scan tool like FIXD that is able to read live engine data.

In the FIXD app, go to the Toolbox and select Live Gauges. From there, set up gauges to monitor any fuel trim parameters that you see available for your car as shown in the image below.

measure fuel trim with FIXD app

What do these numbers mean?

The Short Term Fuel Trims are the adjustments the engine is making right now, a truly live display. The Long Term Fuel Trims are based off the short term data, but over a longer period of time. This makes it easier to see trends in your air/fuel mixture.

The numbers indicate a percentage of how much adjustment is necessary to bring the fuel levels as close to perfect as possible.

A positive reading of 0.0% or above means more fuel is being added to the air-fuel mixture to compensate for a lean condition, while a negative reading (<0.0%) indicates that the ECU is taking away fuel from the mixture to compensate for a rich condition.

Short Term Fuel Trim normally runs between -10.0% and 10.0%, and Long Term Fuel Trim levels typically run between -5.0% and 5.0%.

When viewing Short Term Fuel Trim, you should see your data reading shifting rapidly between rich and lean.

The Long Term Fuel Trim reading will appear more stable. The example above shows a properly running engine, with only minor trim adjustments necessary that are well within normal parameters.

If your trim readings are consistently negative, it means your engine is running too rich, and the computer is constantly trying to add less fuel to get the ratio back where it should be.

A rich condition means that the fuel inside the cylinder isn’t getting completely burned, which indicates an ignition issue. Check out “Are your ignition coils loose or damaged?” for further diagnosis of your ignition system.

If your trim readings are consistently positive, you have the opposite issue. The
engine is running lean, and the computer is trying to add more fuel to achieve the
correct fuel/air ratio. This indicates that the engine may not be getting enough fuel.

Check out “Is your engine getting enough fuel?” below to dive into troubleshooting your fuel system.

Use FIXD to measure your fuel trims!

Get FIXD today for just $19.99 and use it to measure your fuel trims and FIX your P0300 code! Plus, if you’re not sure you measured the fuel trims correctly or simply want advice, you can call our FIXD Mechanic Hotline for help! It’s free for 14 days as part of your FIXD Premium trial. So what are you waiting for? Click here to get FIXD and get back on the road fast!

Do you have loose or damaged wiring at the ignition coils?

Difficulty Level: Beginner

The most likely cause of a misfire is a bad electrical connection. This prevents the spark plugs from sparking at full power. Poor or no spark will not ignite the fuel/air mixture inside the cylinder, causing a misfire to occur.

Check spark plug wires

The ignition coil or coils (pictured below) turn your car’s normal 12-volt power into a high voltage that will jump across the gap in your spark plugs. Some vehicles have one or two coils that send power through the spark plug wires to multiple cylinders. These wires should typically be replaced at the same time as your spark plugs.

If you want to be sure your spark plug wires are still good, watch the video below to see three different methods of testing them.

Check coil-on-plug ignition wires

Check your owner’s manual to see if you have a coil-on-plug ignition (pictured below).

In this type of ignition, small coils attach directly to each spark plug. These eliminate the need for spark plug wires. However, they still have wires that connect to the engine computer and other parts of the vehicle. Make sure these wires are in good condition before moving forward and check for the following:

  • Damaged or missing insulation (outer coating is missing)
  • Broken wires (mice and other small animals sometimes chew on these if they get into your engine)
  • Make sure connectors are tight (no budge or movement)
  • Check for loose wires near the connectors
  • Check for loose engine ground wires 

Have a FIXD mechanic help you find wiring issues

If you’re not sure you found a wiring issue or need advice, a FIXD Mechanic can work with you to make sure you don’t have an ignition issue. You can call FIXD mechanics for free with FIXD Premium. Click here to get FIXD for just $19.99, including a free 14-day trial of FIXD Premium!

Are your spark plugs still good?

Difficulty Level: Beginner/Intermediate (you’ll be removing all spark plugs)

Tools Needed: 

  • Compressed air
  • Spark plug socket

Spark plugs ignite the fuel/air mixture inside the cylinders and make the engine run. While sometimes a misfire can happen because the spark plug isn’t getting enough electrical power to do its job, the problem can also be the spark plugs themselves.

They get old and worn out over time, and need to be replaced regularly. Failed spark plugs can also cause a misfire and code P0300.

Check your spark plugs

To check your spark plugs, you’ll need to remove them from the engine. In the case of a P0300 code, where you don’t know which cylinder the misfire is occurring in, you should check them all. Remove, inspect, and replace or reinstall the plugs one at a time to make sure you don’t get the firing order mixed up. 

First, remove the spark plug wire or coil pack from the top of the plug. Before
attempting to remove the spark plug, use compressed air to blow any dirt or debris out of the cylinder head area. This will prevent any dirt or debris from falling into the cylinder or into the spark plug threads. 

Then use a spark plug socket to remove the spark plug. These specialty sockets feature a rubber insert to hold the spark plug during removal and reassembly. You may need to use a socket extension if the plug is buried deep down in the cylinder head. Try to keep the ratchet and/or extension as straight as possible to avoid breaking the top of the spark plug off.

Signs of a bad spark plug

If your spark plugs look more like the one on the right than the one on the left, you should replace them, regardless of whether they’re causing your misfire or not. A bit of brown or grayish-tan deposits on a used plug is perfectly normal. If this is all you see, you can reinstall the old plug.

Look for these symptoms:

  • Black, dry soot: This indicates a carbon-fouled plug, caused by a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture, or idling your vehicle for too long. 
  • Black, oily deposits: Oil could be leaking into the cylinders. This should never happen, so you may have bigger problems than just a misfire. 
  • Wet: This is typically unburned fuel on the electrodes. This usually happens when the engine is flooded after several unsuccessful starting attempts. 
  • Blisters on the insulator tip, melted electrodes, or white deposits: This is a burned spark plug, which happens when it runs too hot. Causes can include the engine overheating, incorrect spark plug heat range, a loose spark plug, incorrect ignition timing, or too lean of an air/fuel mixture. 
  • Worn electrodes: If the electrodes look smaller than they should be, the spark plug has simply worn out and should be replaced. It is likely that no other problem exists. 
  • Broken electrodes: If the tip has broken off or been flattened, it could be because they piston has hit it inside the cylinder. You may have the wrong size spark plugs, allowing the tip to sit lower than it should. Regardless, this is bad, and your should replace the spark plugs with the proper size ones.
For more on identifying a fouled spark plug, watch this video:

Have a FIXD mechanic help you identify spark plug issues

If you’re not sure your spark plugs are bad or need advice, a FIXD Mechanic can work with you to make sure your spark plugs are functioning properly. You can call FIXD mechanics for free with FIXD Premium. Click here to try FIXD for just $19.99, including a free 14-day trial of FIXD Premium!

Is your engine getting enough fuel?

Difficulty Level: Intermediate

If you have determined that your ignition system is working correctly, the problem may be in your fuel system instead. Just like with insufficient spark, if there isn’t enough fuel in the cylinder, misfires can occur. This is a little more tricky to check than spark, but definitely possible for a DIYer. The first thing we want to check is the fuel pressure. Low fuel pressure can cause intermittent misfires on multiple cylinders, and a P0300 code. When the pressure is too low, the engine does not receive the proper amount of fuel and will start to lean misfire. 

First, check your fuel pressure

If your fuel pressure is low, your fuel pump may be failing, or you may have a clogged fuel filter. Another possibility is that your fuel pressure regulator may be clogged or has failed.

Check out this video to see how to check your fuel pressure:

Next, check your fuel injectors

After determining that your fuel pressure is good, the next step is to check your fuel injectors. 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. The best way to check your fuel injectors is by using a digital multimeter, as demonstrated in this video:

If you don’t have a multimeter, you can get an idea whether they’re working properly or not just by listening to them. Watch this video to learn what to listen for:

Have a FIXD mechanic help you detect fuel system issues

If you’re not sure you found a fuel system issue or need advice, a FIXD Mechanic can work with you to pinpoint the problem. You can call FIXD mechanics for free with FIXD Premium. Click here to get FIXD for just $19.99, including a free 14-day trial of FIXD Premium!

Are mechanical problems causing your misfire?

Perform an engine compression and leakdown test

Difficulty Level: Intermediate

Tools needed: Compression tester, leakdown tester, owner’s manual

If the ignition system and fuel system check out, the next step would be to run an engine compression test and leakdown test to see if there are any mechanical
problems causing your misfire.

At this point, you probably have some big problems inside your engine, and it’s unlikely you’ll be able to fix them yourself. But by doing
these final tests, you can at least get an idea of what the actual problem is, and
what it’s going to take a shop to get your car running again.

Just as important as spark and fuel is the compression of the air inside the cylinder.

As the piston goes up, it squeezes the air inside to many times its normal pressure in the open air. This, plus the ignition of the air/fuel mixture, is what pushes the piston back down.

If there’s no compression, this won’t happen, and that cylinder won’t produce power the way it should.

How to perform a compression test

A compression tester (pictured above) measures how much compression exists in the cylinder when
it’s at its maximum.

Remove the spark plug, then screw the tester into
its place. Crank, but don’t start the engine (you can often remove your fuel pump fuse to do this), and watch the gauge to see the maximum pressure. At minimum, each cylinder
should read over 100 psi.

Check your vehicle’s service manual to find out
the optimum pressure for your particular car. More important than maximum pressure, though, is how each cylinder’s pressure compares to the others.

For example, if you have a four-cylinder engine with compression readings of 180, 165, 30, and 170, the cylinder reading only 30 psi is almost certainly your problem. A little variation
between cylinders, up to 10 percent, is normal and perfectly fine. We’re
looking for one or more cylinders that are drastically out of line with
the others.

How to perform a leakdown test

A leakdown test also deals with compression, except in this case it measures how quickly a cylinder loses compression.

A leakdown tester (pictured above) contains two gauges and attaches where the spark plug normally goes. It also attaches to an air compressor, and blasts pressurized air into the cylinder.

Rotate the engine until the piston is in the “top dead center” position.

This is the top of its stroke inside the cylinder, and during the ignition phase of the combustion cycle where both the intake and exhaust valves are closed.

Add at least 100 psi of compressed air into the cylinder. One gauge will give you the pressure reading, just like a compression tester. The second gauge will read the percentage of how much air is escaping from the cylinder.

No engine is perfectly sealed. A good, healthy engine will still read between five and 10 percent pressure loss. Twenty percent isn’t terrible, but indicates that you may run
into bigger problems down the road. Thirty percent or more is a sign of major problems inside the cylinder you are testing.

Once again, you’re looking for one or more cylinders that are far out of whack with the others. In an older engine, you may read 15 percent across most cylinders but 40 in one. That cylinder is likely your problem.

What problems could cause a lack of compression and 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

Diagnosing which of these problems it is and making the appropriate repairs is likely beyond the ability of most home DIYers. Most of these require partial or complete disassembly of the engine. In this case, we recommend scheduling an appointment with a trustworthy and RepairPal Certified shop in your local area.

Get expert help with mechanical issues from a FIXD Mechanic

If you want to attempt these advanced repairs yourself, a FIXD Mechanic can walk you through it. You get full access to our FIXD Mechanic Hotline as part of your free FIXD Premium trial. Click here to get FIXD for just $19.99, including your free 14-day trial of Premium! 

How To Fix P0300

Before you fix P0300, make sure you correctly diagnose the cause of the code first. If you don’t, you could end up spending $1000s trying to fix the wrong problem. Once you’ve determined the cause of your P0300, click on the sections below to learn more about each possible repair.

How to replace spark plugs and wires

Difficulty Level: Intermediate

Tools Needed: 

  • ratchet
  • extension
  • spark plug socket
  • spark plug gap gauge
  • spark plugs and spark plug wires for your vehicle

Replacing the spark plugs on a car is an intermediate level task for most DIYers. While beginners may require assistance, it’s a relatively simple task that even those with little car repair experience can complete successfully in an hour or less. Watch the video below to see how it’s done.

If you need advice, a FIXD Mechanic can work with you to replace your spark plugs. You can call FIXD mechanics for free with FIXD Premium. Click here to get FIXD for just $19.99, including a free 14-day trial of FIXD Premium!

How to replace a fuel filter

Difficulty Level: Intermediate – Advanced, depending on your vehicle

Tools Needed:

  • Drain pan
  • Fire extinguisher
  • Garden hose and water source
  • Needle-nose pliers
  • Safety glasses
  • Slot screwdriver
  • Vehicle ramps (if filter mounted beneath vehicle)

Yes, you can replace the fuel filter yourself. But how easy it is depends on where it’s located on your specific vehicle. Watch the video below to see how it’s done.

If you need advice, a FIXD Mechanic can work with you to replace your fuel filter. You can call FIXD mechanics for free with FIXD Premium. Click here to get FIXD for just $19.99, including a free 14-day trial of FIXD Premium!

How to replace your fuel pump

Difficulty Level: Intermediate

Tools Needed:

  • Fuel pump assembly
  • Wrench set
  • Jack and jack stands
  • Gasoline container
  • Hand pump

You can save a couple hundred bucks by replacing your fuel pump yourself. However, if you don’t already own the tools, you may spend more preparing the parts for this job than the cost of labor at a mechanic’s shop. It’s also important to know if you’re comfortable working on your car to this level. Improperly installing a gas pump can damage your vehicle.

If you decide to take on your own fuel pump replacement, be sure to keep your owner’s manual and repair guide handy. Not every make and model uses the same fuel pump and setup, so it’s important to know what you’re dealing with before you start taking your vehicle apart. Watch the video below to see how it’s done.

If you need advice, a FIXD Mechanic can work with you to replace your fuel pump. You can call FIXD mechanics for free with FIXD Premium. Click here to get FIXD for just $19.99, including a free 14-day trial of FIXD Premium!

Try FIXD For Just $19.99 Today!

With the FIXD Sensor and app, you can see what your check engine light means, how to fix it, and see how much it should cost to fix. Click below to get FIXD, including a 14-day trial of FIXD Premium, for just $19.99 and save $1000s on repairs!

  • Read and clear check engine codes

    Forget wasting hours Googling engine fault codes. FIXD gives a detailed report of what’s wrong in simple terms, right on your smartphone, and lets you clear codes with the touch of a button.

  • View live OBD2 data

    Ready to maximize your savings with DIY repairs? Use FIXD to view live OBD2 data to test O2 sensors, pinpoint problems and fix them like a pro.​

  • Talk to a certified mechanic

    You'll also get access to our Mechanic Hotline, where an ASE-certified mechanic is available 5 days a week to offer you trustworthy, professional advice and walk you through DIY repairs.

  • Know how much repairs will cost

    With Confirmed Fix & Cost, you'll see the most likely repair for your car's check engine light, plus how much it should cost. Fix it yourself or take it to a shop knowing you can't get ripped off.

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    Hate surprise repairs? Our Issue Forecast feature can predict which problems your vehicle is most likely to have in the future so you'll be prepared.

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    Save time and money and be confident you’ll pass emissions testing the first time with our at-home Emissions Precheck.

Real reviews from real drivers

Sarah B.
Sarah B.
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The main use is the cost estimates for everything. It’s powerful to be able to hold that up to the mechanic and be like ‘wait a second, that estimate is way over.'
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Jennifer G.
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My family has saved thousands of dollars in labor alone. Thank you! Over 6 different vehicles have been fixed using FIXD Premium.
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Travis W.
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Love FIXD Premium. Went to a mechanic and he told me I needed much more done to my Hyundai and I told him to fix only what the app said to repair. He then asked how I know, I showed him the FIXD recommended repair and cost. Never went back.
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This has been a big money saver for me. I was having car problems and I took it to the shop. $500 to fix the car. I decided to buy this before doing the repairs and glad I did. The problem was just a mass air flow sensor. Part was only $90 and I went on YouTube and fixed it in 15 minutes with a screw driver.
Noel Williams
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If you own a vehicle you absolutely have to get one, I put in the info and it told me exactly what the problem was and rated the seriousness of it, and provided locations where I could get it fixed without getting overcharged.
C. Pereira
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Easily top 3 if not top product I have ever purchased on Amazon. Easy to set up, install, and use. The premium service is excellent with a 24/7 mechanic helpline (who went above and beyond). I had a check engine light come in with a throttle control motor specifies as the issue. In total I spent $50 for the Device, $9 for a month of premium, and $20 on a relay. Problem solved (after talking it through with the call in mechanic). I spend $79 total, or could pay $120 for a diagnostic, and still not resolve my issue. 100% would purchase again and recommend.
Gretchen M.
Gretchen M.
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I called the mechanic hotline and believe you me, my money... was well spent. I spoke to John and he was a gentleman. Didn't talk down to me... answered my questions and told me to never hesitate to call for any reason about the car. Thank you so much for the ability to call a mechanic when I need help. I appreciate it and believe you me I will tell everyone what a class A company FIXD is.