How to Diagnose and Fix a Cylinder 3 Misfire (P0303)

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Narrow down the symptoms and solutions to fix your engine issues
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If your OBD-2 scanner has pulled up a cylinder 3 misfire error and you see the P0303 code, it indicates that one of the cylinders in your engine isn’t combusting fuel properly. This is one of the most notoriously confusing errors to diagnose and clear, but if you’re patient and take your time there’s a good chance you’ll be back on the road in no time. Here, we’ll walk you through what this code means, why it’s significant, and what you can do to diagnose and fix the issue.

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Section 1 of 5:

What does P0303 mean?

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  1. A misfire occurs when one of the cylinders inside of your engine doesn’t fire because something is off in the fuel-air-spark ratio. In the case of P0303, your engine control module (ECM) or powertrain control module (PCM) has detected multiple misfires in your 3 cylinder. [1]
    • On an inline engine, start counting the cylinders with 1 at the front, closest to the timing cover.
    • On a V-engine, cylinder 1 is slightly in front and at the front of the engine. It’s in the bank that sticks out in front of the other bank. The 3 cylinder will be next to the 1.
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Section 2 of 5:

Symptoms

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  1. As soon as your ECM or PCM registers a misfire, your check engine light should pop on. It may stay solid, or it may begin flashing to indicate there’s a catastrophic loss of power. In any case, this is major sign something is wrong with your vehicle. [2]
    • On some newer vehicles, the check engine light will go from orange to red instead of flashing.
    • If you’re driving when a check engine light comes on, pull over and contact a tow truck if you want to avoid potentially destroying your engine.
  2. Your 3 cylinder is misfiring, which means it isn’t generating power the way that it should. On a 6-cylinder engine, this means that you’re only getting about 80% of the power you should be getting. At the least, you’ll notice a loss of power. If the absent power is essential enough, it will also cause jerking as you drive, or worse, stalls. [3]
    • This is why your engine may get damaged if the problem goes unattended—the entire engine has to work so much harder to maintain speed, which causes serious strain.
  3. Without the 3 cylinder, your engine isn’t going to run the way it is designed to run. You’re going to end up burning through more gas than you should under normal conditions, so you’ll burn through the current tank much faster than you’d expect. [4]
    • This is important to keep in mind if you’re already kind of low on gas and the check engine light pops on suddenly. You probably don’t have nearly as much time to get to the mechanic as you think.
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Section 3 of 5:

Can I drive with a P0303 error?

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  1. If you can control your vehicle and you’re going to drive immediately to the mechanic, go for it. But don’t ignore the check engine light and continue driving. You could seriously damage your engine, and your car could stall out at any moment. If you want to avoid blowing your engine out, pull over and cut the power out. [5]
    • It’s especially dangerous if your check engine light is flashing. This is your car’s way of telling you that your vehicle is not drivable.
Section 4 of 5:

Potential Causes

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  1. Your ignition system is responsible for providing the spark needed to ignite the gas and air inside of your engine. Each cylinder fires independently (but in a carefully measured pattern) when the spark plug transfers the electrical energy from the ignition coil into the combustion chamber. As a result, the problem could be: [6]
    • Bad spark plugs. Specifically, it could be a bad spark plug in cylinder 3.
    • Failed ignition coils. There could be a damaged coil feeding cylinder 3’s spark plug inadequate energy.
    • Damaged spark plug wiring. The plug’s connections and wiring could be faulty.
  2. Your cylinder may also be misfiring because it isn’t getting enough fuel to burn. The spark plug may be igniting just fine and there may be enough air in the cylinder, but there may just not be an adequate amount of fuel. This could be caused by: [7]
    • Bad fuel injectors or fuel pumps. These devices are responsible for sending fuel from the tank to the cylinder, so they may just need to be replaced.
    • Bad readings from failing oxygen sensors. Your PCM or ECM are basically your vehicle’s computer. If they’re getting bad information from a dead oxygen sensor, it could be telling the injectors to provide too much fuel.
    • Damage in the PCM or ECM. How does your vehicle know how much fuel it needs? The ECM/PCM takes measurements and relays that information. If the ECM/PCM is damaged, it won’t relay solid information.
  3. This is the least likely cause unless the engine is on the older side and hasn’t been maintained very well. Cylinder 3 could be misfiring because of physical damage. It’s equally as likely that some component in the transmission or vacuum system is to blame for the cylinder firing improperly. It could be caused by: [8]
    • Vacuum leaks. Any leak behind the mass airflow sensor will allow too much air to enter the engine and mess with the cylinder’s ability to fire.
    • Physical damage in the cylinder, like worn piston rings or cylinder walls.
    • Camshaft, timing chain, or crankshaft failure. These components rotate in tandem with the cylinders, so if either of them are firing improperly the cylinder will misfire.
    • Leaks in the manifold gasket.
    • Compression or exhaust failures. If there are toxic fumes building up inside of the cylinder and they can’t get out, it may force a misfire.
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Section 5 of 5:

Diagnostics & Solutions

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  1. If there are multiple error codes on your OBD-2 reader, the P0303 code could be a byproduct of some other underlying problem—not the main issue itself. For example, if your crankshaft needs to be replaced, the cylinder is just going to keep misfiring (even if you clear the code) if you don’t deal with the crankshaft.
    • Check the freeze frame data in your OBD2 scanner. This will give you the timeline of the error codes. Deal with and fix every error occurring before the P0303 cylinder 3 misfire, first.
    • The freeze frame data might be confusing if this is the first time you’ve looked at it. Basically, it’s a list of all events in your engine. Start at the top and write down every error code or acronym. Then, look online to determine what they refer to. This should give you a list of repairs you need to make.
  2. Disconnect the battery to avoid shocking yourself. Then, remove any spark plug wires or connectors that feed into the engine above each cylinder. Remove the wire plug by gripping it and gently lifting it up. Slide a long socket wrench down the hole into the spark plug housing. Unscrew it and remove the spark plug. Repeat this process for every spark plug and replace them . [9]
    • Always replace each of the spark plugs at the same time. When one goes bad, it’s a sign the others will go bad soon.
    • If you aren’t sure whether the spark plugs are to blame, test the plug in cylinder 3 by swapping it out with the plug in cylinder 4 or 2. If that new cylinder misfires, you’ve confirmed the spark plugs are to blame.
    • A new set of spark plugs is likely to run $100-250, although it may be a bit more if you need new wiring done by a mechanic.
  3. Inspect the ignition coils and replace them as needed . If the spark plugs appear to be fine or the problem persists, your ignition coils are the next most likely culprit. Locate the coils between the battery and the distributor or ECM. Swap the cylinder 3 coil and cylinder 4 coil, and start the engine. If cylinder 4 misfires, replace the bad ignition coil and reassemble the spark plug assembly.
    • If you have an ignition coil railing where they’re all connected, unlock the railing and replace the entire system.
    • You can test the ignition coils by performing a spark test or by using a multimeter if you’d like.
    • If you go to a mechanic, ignition coil replacements will cost $200-700 depending on your car's make and model.
  4. If you can’t find the problem in the spark plugs or coils, go around your engine bay and inspect all of the cables and connections. Make sure everything is plugged in entirely and replace any frayed cables or blown fuses. If you’re especially handy and know how to work on your vehicle, you could also try: [10]
  5. There’s only so much that you’ll be able to do on your own, and the P0303 error code is notorious for having a ton of potential causes. If you still can’t identify the cause of the misfiring cylinder, tow your car to a mechanic. They’ll be able to isolate the problem and fix it. [11]
    • The cost of these repairs by a mechanic will vary depending on the specific components you need to change. Expect to spend anywhere from $250-1,200. The most expensive repair will be a new fuel pump.
    • Vacuum leaks are also relatively pricey. These could run $150-1,000.
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      References

      1. ↑ https://www.dmv.de.gov/VehicleServices/inspections/pdfs/dtc_list.pdf
      2. ↑ https://www.dispatch.com/story/business/2015/09/26/car-talk-despite-mistrust-get/24013737007/
      3. ↑ https://cartreatments.com/engine-misfire-symptoms/
      4. ↑ https://cartreatments.com/engine-misfire-symptoms/
      5. ↑ https://www.dispatch.com/story/business/2015/09/26/car-talk-despite-mistrust-get/24013737007/
      6. ↑ https://www.dispatch.com/story/business/2015/09/26/car-talk-despite-mistrust-get/24013737007/
      7. ↑ https://www.dispatch.com/story/business/2015/09/26/car-talk-despite-mistrust-get/24013737007/
      8. ↑ https://www.dispatch.com/story/business/2015/09/26/car-talk-despite-mistrust-get/24013737007/
      9. ↑ https://static.nhtsa.gov/odi/tsbs/2019/MC-10161643-0001.pdf
      1. ↑ https://static.nhtsa.gov/odi/tsbs/2019/MC-10161643-0001.pdf
      2. ↑ https://www.repairsmith.com/blog/mechanic-shop-near-me/
      3. ↑ https://static1.squarespace.com/static/59970b344c0dbfc24e10d9a5/t/59c9637f32601e31bf2c3f68/1506370436958/MASS-Module_4-2012_Update.pdf
      4. ↑ https://www.caranddriver.com/features/a25063304/reset-check-engine-light/

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