How to Fix P0303 Code Cylinder 3 Misfire

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If you're reading this, you may have detected a P0303 code on your car. It is one of the most common codes, and it means that cylinder 3 is misfiring. This article will cover how to fix the P0303 code cylinder 3 misfire to save some money on expensive repairs by doing it yourself at home.

What does the P0303 code mean?

an open car hood

Code P0303 is a generic powertrain trouble code that warns you of a failed cylinder 3. While it may only mean a faulty spark plug or spark plug wiring, it may also suggest other air/fuel mix issues. 

It is best to use an OBD2 scan tool to diagnose the issue properly. Also, check your user manual to know the placement of your cylinders and what number is designated for each.


  • Illumination of the check engine light
  • Jerky acceleration
  • Hard starting and stalling
  • Fuel fumes from the exhaust
  • Reduced engine power
  • Poor gas mileage
  • Engine misfires
  • Poor driveability


  • Faulty spark plugs
  • Faulty wirings of the spark plugs
  • Loose connection with the fuel injector circuit
  • Issues with fuel injector driver
  • Worn piston ring
  • Leaks on the vacuum
  • Low fuel pressure
  • Defective camshaft or crankshaft sensors
  • Leaking head gasket
  • Low engine compression
  • Poor quality of fuel

Is the P0303 code serious?

Yes. Failure to start is the most common effect of failure to fix the P0303 code. However, it may lead to more severe damage to the catalytic converter and other unsafe conditions while driving.

How do you diagnose the P0303 code?

a Bluetooth adapter for OBD2 apps

To accurately diagnose why your check engine light is on, use an OBD II scanner or OBD II app. The cylinder 3 misfire detected may be caused by other issues, so it is best to rule out other problems.

  • If the misfire happened for the first time, try clearing the OBD2 codes on your car, then do a test drive. If the check engine light does not turn on again, it may be a random misfire and need no action.
  • Check the following: vacuum hoses, ignition coils and waiting, spark plugs, fuel injectors. Use a digital multimeter to see if they are working.
  • For the fuel pressure, use a fuel pressure gauge to rule out this trouble.
  • Do a compression test to rule out issues like skipping or misalignment of the timing chain.

How can you fix the P0303 code?

Before you do any check on your car, be sure to disconnect the battery first. Do not let the positive and the negative terminals touch a metal tool so it won't cause a short circuit.

Replace Damaged Wirings of the Spark Plug

spark plugs and spark plug wrench

  • If your issue is loose connections on spark plug wires, tighten them up and test your car again. If damaged wirings caused the trouble code, replace them.
  • Measure your spark plug wires and cut them an inch longer to give you more freedom to work with them.
  • Push the end of the wire in until you hear a faint pop.
  • Do not remove the wirings all at once. Instead, identify their connections and change one wire at a time.
  • Remember to check your spark plug as well and clean it of carbon or soot. Use a multimeter to measure its output. If it does not respond, replace the spark plug, too.

Replace the Spark Plug

a mechanic cleaning a used spark plug

  • Even if your spark plug still works, it is best to replace them every 1,000 miles traveled.
  • Remove the spark plug wirings. Be sure you know where to replace them later.
  • Remove the coil-on-plug (COP) and check for damage. Damaged COP may also cause a misfire.
  • Loosen the spark plug to remove it. Avoid thread damage by blowing into the spark plug hole to remove any dust or particles that may damage the thread.
  • Install the new spark plug. Screw it in the opposite direction to tighten it well.
  • Replace the COP and the wirings. Reconnect the battery, then do a short test drive.

Fix or Replace the Fuel Injector

a fuel injector

If you can still fix the fuel injector, you may try to remove and unclog it with a fuel injector cleaning kit.

  • Choose the suitable cleaning solvents for your car. Check your user's manual for compatibility.
  • Remove the pump and insert a U-tube to let the fuel connect to the gas tank.
  • Remove your car's fuel pressure regulator, then attach the cleaning kit to the compressed air hose.
  • Turn your car on, allowing the cleaning kit to suck in the cleaning solvent, then wait until the engine shuts off.
  • Remove the cleaning kit, then reconnect the fuel pump and fuel pressure regulator.
  • If cleaning does not do the trick, replace the fuel injector.
  • To replace the fuel injector, follow these steps:
  • Remove the bolts or screw that holds the primary fuel rail to disconnect it. Disconnect the wirings first if they run along the fuel rail.
  • Remove the wire to the fuel injector plugs, then pull the fuel injector with a fuel injector puller or flathead screwdriver.
  • Once the fuel injector is out, cover the intake manifold to avoid having anything fall into it.
  • Replace the new fuel injector, then all the other connections you removed earlier.

Fix Low Fuel Pressure

fuel pressure regulator

Depending on the make and model of your car, your fuel pressure may be mechanical or has a sensor. Check with your car dealership to know which one is on your vehicle.

  • Remove the fuel pump driver module, then check it for corrosion, loose wirings, or other visible issues. Use also a digital multimeter to check the voltage. If it is not within specifications, replace it before reconnecting everything to avoid fuel leaks or low-pressure issues.
  • Check other causes of low fuel pressure - faulty sensors, dirty filters, clogging - then address them before you decide to replace your fuel pump driver.

Replace Piston Ring

a car cylinder head

If you have the right tools, you can replace your piston rings by yourself. Don't forget to access a service manual to help you along the way too. Check also the head gasket since you may need to replace it as well.

  • Disconnect the wirings connected to the cylinder head to remove it. Drain the coolant and remove the exhaust manifold.
  • Remove the intake manifold, then the rocker arms and pushrods. One valve means one rocker arm and one pushrod.
  • Remove the cylinder head, then clean it well. Prevent debris from falling inside the cylinder head by covering all holes.
  • Pull out the piston, and then the piston rings. Soak the piston in kerosene to remove any gunk, then take it out to air dry in a well-ventilated area.
  • If the piston rings are too hard to remove, let the kerosene do its magic overnight to loosen the rings.
  • Install the new piston rings. Be sure they are inside the ring grooves, then reverse your steps later to assemble the cylinder head and the rest of the block.

Repair Vacuum Leaks

vacuum tester gauge

If air can enter your engine through a faulty gasket or vacuum leak, it may result in the code P0303. Find the vacuum leaks with a vacuum tester gauge.

You may also use a carburetor cleaner, a propane torch, or a water spray bottle. However, for a safer route, we recommend using the spray water bottle if you don't have a tester gauge as the propane torch and carburetor cleaner may cause other damage if you don't know how to control it well.

  • Aside from the vacuum lines, check also on the intake manifold and other components. Tighten your intake and exhaust manifold to be sure.
  • Hook up the vacuum lines to the vacuum tester gauge, pump it, and get a reading. Note which lines you've tested, so you don't have to do it twice.
  • Listen also for hissing sounds under the hood. Depending on the size of the leak, it may range from a low whistle to full-blown hissing.

Use a Better Quality Fuel

Poor fuel quality may also cause cylinder 3 misfire detected and the illumination of the check engine light. Use the recommended fuel for best results.

How much do I need to fix the P0303 code?

Depending on the source of the trouble code, it may be several hundred to several thousand. Here is a rough estimate of replacement and repair of the specific causes of misfires:

  • $100 to $250 to replace broken spark plugs
  • $150 to $250 to fix faulty ignition coil
  • $100 to $300 to replace spark plug wires
  • $275 to $400 to fix faulty ignition coil
  • $200 to $800 to fix vacuum leaks
  • $200 to $1000 to fix bad fuel delivery
  • $450 to $650 to fix faulty valve springs
  • $1500 to $3000 to fix broken piston rings

OBD2 Code P0303 FAQ

What causes misfire on cylinder 3?

A misfire may occur when an insufficient amount of fuel is fully burning in a cylinder. A properly running car needs a spark from the engine to power it, which starts the combustion process that drives rotation and acceleration. The reason for a misfire can be many:

  • Worn-out spark plugs
  • Loose spark plug wiring
  • Defective ignition system
  • Dirty or blocked fuel valve
  • Mechanical failure within the engine

Can a bad catalytic converter cause a P0303 code?

No, but this code can cause damage to the catalytic converter.

Can I drive with a P0303 code?

You can, but we recommend that you fix this trouble code, especially if you've not pinpointed the culprit before it causes more damage to your car and endangers your life. A cylinder may give out while you're on a long drive and result in a car accident.


The code P0303 is not a big issue if you fix it immediately and take extra precautions to prevent its occurrence. You should always use an OBD2 scan tool to diagnose the problem and read your user manual if you are unsure what engine cylinders correspond with which number. 

What is your experience when dealing with a cylinder 3 misfire? Let us know in the comments below!