P0303 – Meaning, Causes, Symptoms, & Fixes

Code P0303 Definition

Cylinder 3 misfire detected.

Commonly associated with codes: P0300

What Does P0303 Mean?

P0303 indicates that cylinder number 3 is experiencing misfires. A misfire occurs when an insufficient amount of fuel is burning in a cylinder. The efficient burning of fuel is essential to engine operation as the combustion of fuel is what provides the energy to power the engine. A misfire from one or more cylinders can be caused by many reasons from a faulty ignition system, fuel system, or internal engine failure. When P0303 occurs, it should be fixed immediately as long term driving with engine misfires could cause consequential damage to your engine.  Many times, P0303 occurs when there are worn-out spark plugs, spark plug wires, or a faulty ignition coil.

What Are the Symptoms of Code P0303?

  • Check Engine Light is on
  • Check Engine Light flashing
  • Engine runs rough and shaking
  • Lack of power from the engine
  • Fuel smell from the exhaust
  • Hesitations/Jerking when accelerating

What Is the Cause of Code P0303?

(* = Most Common)

  • Faulty or worn spark plugs *
  • Faulty spark plug wires or coils *
  • Distributor failure *
  • Faulty fuel injector
  • Vacuum leak
  • Low fuel pressure
  • Camshaft sensor defective
  • Crankshaft sensor defective
  • Engine timing off
  • Leaking head gasket
  • Low engine compression
  • Poor quality fuel

How Serious Is Code P0303? – Severe

P0303 should be repaired immediately. Ignoring this error could result in ignition failure, catalytic converter damage, and unsafe/dangerous conditions while operating the vehicle.

Code P0303 Common Diagnosis Mistakes

Loose-fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked.

Tools Needed to Diagnose Code P0303:

How To Diagnose Code P0303:

Difficulty of Diagnosis and Repair – 2 out of 5

  1. Use FIXD to scan your vehicle to verify P0303 is the only code present. If other codes are present, they must be addressed first.
  2. Check for loose connectors at the ignition coils or for damaged wiring (specifically cylinder 3). Look for loose engine ground wires as well. These can cause random misfire conditions. Tighten or connect where necessary.
  3. Check the condition of your spark plugs and spark plug wires. If your vehicle is equipped with individual coil packs instead of spark plug wires, start by removing the cylinder 3 coil and swapping it with the cylinder number 4 coil. If the misfire moved to cylinder number 4 (P0304), then you have determined that the coil pack is faulty and needs replacement. You can do the same test with the spark plug, i.e., move cylinder 3 plug to say cylinder 2 and if the misfire is now P0302 then the plugs are at fault. Replace spark plugs and spark plug wires/ignition coil packs if needed and recheck for misfires. (How to identify a fouled spark plug, How to test spark plug wires, How to gap spark plugs, How to replace spark plugs, How to swap ignition coils)
  4. 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. (How to check fuel pressure). Check that 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).
  5. If the ignition system and fuel system checks out, you may want to perform an engine compression test and leak down test to see if there are any mechanical problems causing your misfire. Some common mechanical problems that cause misfire can be:
    1. Leaking head gasket
    2. Broken valve spring
    3. Broken piston ring
    4. Worn valve guides
    5. Burned valve
    6. Timing chain or belt skipped tooth and engine is off time.

Estimated Cost of Repair

For error code P0303, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair. 

  • Spark plugs $40-$640 (some cars require Intake manifold removal)
  • 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

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Comments 25


August 8, 2018

Hi after fixing IAVC i started seeing P0303 & P1399, i replaced complete tune up system ( spark cable/plugs/router) can you help what else to fix


June 23, 2019

96 Chevrolet Silverado 5.7L vortec po303 cod new distributor, coil ,plugs ,wires, distributor cap,rotor button, crank sensor, fuel pump, and spider injectors. Still skipping and running rough when driving. Idles perfect ,and fine when you put it in neutral while driving. Only does it when driving. What else could it be please help

Paul Wittmann

August 20, 2018

Greatest device on the market.

FIXD Marketing

September 3, 2018

Thanks Paul! We appreciate it!


April 10, 2019

Replace camshaft sensor check for loose wires or corosion. And you didn’t mention replacement of the coil which is the main ignition component… I found a broken connection on my 05 freestar 4.2 doing this tomorrow. Hope this helps


November 9, 2018

I have a 07 grand Prix it keep say reduce engine power I have changed the map sensor,throttle body,gas pedal


November 29, 2018

Everything was fine until this morning when I started the vehicle. I bought gasoline last night at a station I dont generally go to and bought ethanol which I dont usually do. Today I showed a P0303 and I think it is bad gas causing it but unfortunately I have a full tank now. wondering if I put some octane boost or some stuff to rid gas of moisture would help

Chad Lucas

December 16, 2018

Water in gas I believe in going threw the same issue I’ve added some water remove idol seems to be getting smother


December 8, 2018

While driving my 1998 Navigator, the check engine light started to flash then stayed steady.
I ran a scan and found out I have a misfire in cylinder 3. I have the device turn off the light and then I rerun the scan- no issues. I reran the scan four additional times and still no issues and no check engine light.
I do not understand the reason for the issue but this little device keeps my blood pressure low!

FIXD Marketing

December 9, 2018

Glad we could help Michael! Always a bonus when we can contribute to the health of the people who use FIXD!


Anthony M

December 22, 2018

Just had my valve cover gasket changed. Other than a leak there was nothing wrong, no misfires no bucking, no check engine light nothing. The mechanic gives me the car and I drive five minutes down the street and the check engine light goes on, car starts to buck a little. I go right back to the shop. They connect the computer an tell me it’s a coil. They have to take off the manifold again to change the coil. Now they tell me I owe for the coils in the back and that there was oil in the wells and one coil was split.

My question: if the coil was split and oil in the well, wouldn’t the light have been on and my engine misfiring before I brought it in??

I find it hard to believe I wouldn’t have an issue with the coils in that condition and oil in the wells previously.

What are your thoughts.

Colleen Tipple

January 1, 2019

My 2019 Kia Forta’s malfunction light came on when my milage was only at 811. I took it to the dealer; it was a PO 303 and they replaced the necessary components. Is this common on a new vehicle?


January 14, 2019

Thanks should NOT be happening on a brand new vehicle. Take it back to the dealer.

Christopher Warneke

January 12, 2019

I have a 2015 kia forte 1.8 and I started with codes indicating that cylinder 4 was misfiring after changing all plugs and checking for resistance in all 4 coil packs I now have codes indicating that cylinders 1,3 and 4 are misfiring. Please if you have any idea why this is happening I would greatly appreciate any help you can offer.


May 4, 2019

Vacuum leak,intake leak,fuel pressure, check timing,then head gasket. If car ha 200,000 miles, run some fuel booster through it, then start with vacuum/ intake leak,then fuel pressure,then compression test.


January 15, 2019

So the app and everything is great. But I’m still getting the codes for misfires and I’ve changed the spark plugs the ignition coils and the egr too. Can anyone point me in the right Direction cause I’m lost on how to fix it at this point.


January 24, 2019

This code can be tripped when travelling thru deep water and the plugs get soaked,leading the spark to go to ground rather than create a spark internally.

Donald Simmons

February 22, 2019

Hell my name donald. I have a Chrysler 200 s. Its I bought this car around this time of last year with 2500 miles on it. Since i had it ,all i change was the oil. My wife drives the car the most cause i work many months out of state. I haven’t
Did anything else on it yet until now😣😣. I took it to advance auto parts
Different codes popping up saying, it’s the throttle body sensor peddle position sensor. When u mash the peddler, it takes like 2 seconds before take off


March 28, 2019

I have an 08 Ford f150 5.4 3v V8 Triton. I just changed all 8 new plugs and 8 new coil packs. I am getting a P0300 P0303 and P0308 which I know are all cylinder misfires and a solid at times and blinking at times check engine light. Why? Before I put the new ones on never had a light on but occasionally the shuttering. I did the cool pack swap and the codes did not follow the could packs they stayed in the same cylinders. I am lost as to what is next or why I am having these misses. Help please.

Rick Lopez

May 13, 2019

Replaced points and plugs on 2002 Toyota Tundra V8 4.7L and engine still misfiring. Notice primarily while stopped and runs fairly smooth while driving. Replaced oxygen sensor. Engine light flashes for a bit then stays on. Any thoughts? Fuel pump? Injectors? Thanks.


May 14, 2019

i have a 2014 dodge dart showed thep0303 code but car wont start. Could this be just a fuel filter or is this more serious

James H. Gladbach

July 15, 2019

No answer why it starts misfiring after it has been driven about 100 miles steady.


July 19, 2019

Pretty neat little tool. Gave me a trouble code and cylinder # for a misfire which I already knew but had no way of diagnosing which cylinder it was. Through a process of elimination I should be able to correctly make this repair on 13 yr old vehicle with 148k miles on it.


September 5, 2019

My 02 Honda Civic was having random misfires when it was first cranked in the morning and when I had to accelerate quickly In traffic ran the codes came up with Cylinder Misfire #3 I change the ignition coil to no luck I simply changed the spark plugs and that was is thanks for the information it helped a lot !


September 23, 2019

I have an 05 Nissan Murano my check engine stays on and sometimes it blinks and when it does my car starts to sputter. It idols rough like it’s going to die acceleration is weak. What should I look for first. I don’t have much money to just throw parts at it. Please help.

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