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Watch the video below then follow the steps to diagnose and fix your P0303 code! If you need any help, start your free FIXD Premium trial to access our Mechanic Hotline!
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While check engine lights are never fun, P0303 is among the most common diagnostic trouble codes drivers will experience. The cost to fix this code at a repair shop can range from $250-$1900 on average. 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 P0303 (Cylinder 3 Misfire), you need to identify the root cause of the misfire. The most common cause of a P0303 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.
FIXD Premium members can see the most likely repair and cost for their exact car in the FIXD app. If you want to see your most likely repair and cost, access our Mechanic Hotline, and more, start your free FIXD Premium trial today.
Difficulty Level: Beginner
Tools Needed: OBD2 Scanner
P0303 may not be your only code. Before you fix anything related to P0303, 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. There are a few options for this:
Pros: The main benefit of this is that you get your answer for free. They will scan your car and let you know all of the codes on your car.
Cons: The downside is they won’t tell you what is causing your error codes. You’ll still have to go to a mechanic or figure that out for yourself. If you want them to clear your check engine light, they might not be willing to do that for liability reasons.
Pros: A mechanic will be able to scan your car and will be able to figure out what is the root cause of your problems and fix it. Mechanics usually charge about $100 for a diagnosis, but they typically deduct this cost from your bill if you get them to fix the problem too.
Cons: This is probably the most expensive option. Not only will you pay for the parts, but you’ll have to pay for labor.
Pros: With FIXD you’ll be able to scan your codes yourself. FIXD tells you what your code means in simple terms, how serious it is, and advice on how to fix it. You’ll also be able to read live data from your car’s sensors to diagnose your problems. If you need any help or expert advice you can call the FIXD Mechanic Hotline for free to get specific answers for your car’s problems.
Plus, with FIXD you can clear your codes with one click. This is important if you want to test whether or not your repairs worked, or if you know your issue isn't serious and want to turn off the check engine light.
Cons: The FIXD Sensor isn’t free (it's $19.99), but you can sign up for a free 14-day trial of FIXD Premium. This should be enough time to fix your car with the help of our on-call mechanics. Click here to start your free trial!
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.
You can determine whether your misfire (p0303) 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.
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.
If you’re not sure you measured the fuel trim correctly or simply want advice, a FIXD Mechanic can work with you to find a leak. You can call FIXD mechanics for free when you’re a member of FIXD Premium. Click here to try FIXD Premium for free for 14 days.
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.
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 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:
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 when you’re a member of FIXD Premium. Click here to start your FREE FIXD Premium trial.
Difficulty Level: Beginner/Intermediate (you’ll be removing all spark plugs)
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.
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.
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.
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 when you’re a member of FIXD Premium. Click here to try FIXD Premium free for 14 days!
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.
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:
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 when you’re a member of FIXD Premium. Click here to try FIXD Premium for FREE!
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.
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
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.
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.
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 a Premium member. Click here to try FIXD Premium free for 14 days!
Before you fix P0303, 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 P0303, click on the sections below to learn more about each possible repair.
Difficulty Level: Intermediate
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 when you’re a member of FIXD Premium. Click here to try FIXD Premium free for 14 days!
Difficulty Level: Intermediate
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 when you’re a member of FIXD Premium. Click here to try FIXD Premium for free for 14 days!.
Difficulty Level: Intermediate (with direct fit catalytic converter) – Advanced (with universal fit catalytic converter
Tools Needed: New Catalytic Converter, Socket Wrench, Penetrating Oil
Catalytic converters can be relatively easy to replace depending on what type they are. Direct fit catalytic converters are designed specifically for your car and will bolt right into your exhaust system. Direct fit catalytic converters can be expensive because they are made to fit one model. Universal fit catalytic converters are cheaper and are designed to fit a wide range of cars. You will have to custom fit a universal catalytic converter into your car. If your goal is to save money and you aren’t afraid to work on your car, you should consider a universal fit catalytic converter. Watch the video below to learn how to replace your catalytic converter.
If you need advice or help, a FIXD Mechanic can work with you to place a new catalytic converter. You can call FIXD mechanics for free when you’re a member of FIXD Premium. Click here to try FIXD Premium free for 14 days!
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