Ignition Systems ControlOBD2 Codes

P0303 – Meaning, Causes, Symptoms, & Fixes

P0303 Cylinder 3 Misfire Detected
Reading Time: 4 minutes

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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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43 Comments

  1. 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

    1. 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

      1. Camshaft sensor

    2. Could not also be the battery terminals and grounds need to be brightened up

  2. Greatest device on the market.

    1. Thanks Paul! We appreciate it!

      1. 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

    2. So glad I got this. No idiot lights just hesitation when accelerating. Fixd just saved me from ruining my engine. Thanks.

      1. Thank you so much for the kind words and for using FIXD! We are so glad to hear your engine was saved. 🙂 Happy and safe driving!

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

  4. 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

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

    2. I’d definitely add octane booster as well as Seafoam & Lucas injector cleaner you tank just to be on safe side. But I add Seafoam and Lucas at least every 2-3000 miles. Also Seafoam is compatible and good to put in engine oil as well.

  5. 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!

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

      FIXD Team

  6. 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.

    1. My thought is while the shop was replacing gasket THEY damaged your coils/coils. Of course most mechanics won’t be honest and tell you they did, they just want to make more money off you. Single coil, or individual coils per cylinder? Either way though it’s their fault! Sad but it happens all the time. That’s why I invest in my FIXD sensors for all my rides, plus I have a higher end regular OBD2 code reader. I would NEVER take a car to a shop without performing multiple scans with FIXD and regular code reader. I instead invest in tools, and a Chiltons(preference) or Haynes vehicle specific repair manual as soon as I buy a vehicle. Only time I’ve ever taken a ride into a shop was AFTER I’d tried EVERYTHING and replaced all possible parts and it was a pre OBD2 port ‘93 Jeep Grand Cherokee. That would run great but had a small short in the pigtail/plug going into coil. That would just kill engine until I wiggled every wire under hood. Sorry for novel….but I took it to Jeep dealer for free diagnostics because it wasn’t giving me a code with the old OBD1 “count the flashes” of check engine light for code. Other than that pre 2010 vehicles are still quite simple to DIY. Just less room under the hood and lots mire wiring! Not like my first 3 ‘72 Chevy trucks I had when turning 16 in 1996.

  7. 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?

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

  8. 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.

    1. 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.

  9. 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.

    1. I have a 1998 f150 5.4lv8 Triton 4×4 and these 10 codes po136,po151 ,po156,p1130, p1309,p0301,po303,po151 and po174 . How do I fix it so I can drive it without it dying

    2. Hi there,
      I have 2008 BMW 116i automatic transmission 1.6 petrol,
      Take a lot of petrol 11 litre per 100k inside town,
      I think 8 litre for 100km fear ,
      Help please?

  10. 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.

    1. I have a 2014 Jeep Grand Cherokee Laredo I just purchased a year ago and. I have only has 61,000 miles on it and I’m already having engine problems it was diagnosed needs a new cylinder head and lifters and Rockers replaced the problem is they don’t have any cylinder heads available to my warranty with mopar parts and there’s a back order on that cylinder head I’m concerned about this problem found out there’s no recall on the cylinder head pots for that model what I was told but I did some research and they did have those types of Jeeps with defects on those pots but of course Chrysler warranty said there was other recalls but not this one I’m already thinking about trading it in for another newer Jeep so what do you recommend I should do?

  11. 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

  12. 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.

    1. I have the same truck with the diagnosis being cylinder 3 misfire. Were you able to fix the problem with yours? If so, what was it? Thank you ?

  13. 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.

  14. 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

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

  16. 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.

  17. 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 !

  18. 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.

  19. Pcv valve on valve cover was messed up. Tried driving it. Missfired like crazy broke 3 of the bottom spark plugs then cute off. Replaced all. Sounds like it wants to start. It’s getting fuel cranks for a second then shuts off. Will not stay crunk. Why did it do that

  20. I have a 1984 Ford Ranger V6 2.8 l it’s Sparks when the motor is not even running as soon as I put the key in the on position I am at a loss any help would be nice

    1. I have a 1984 Ford Ranger I turn the key to the on position and I can hear it sparking there’s a relay that keeps clicking to if the motor is not even turning over it’s a V6 2.9 liter and I have no idea what the problem is

  21. I have swapped spark plugs and coil positions with no luck. Could it be the coil harness?

  22. After taking it to mechanic and replacing spark plugs and coils the check engine light remains on and when I scanned it after the repair it still says P303. Does this mean it potentially is another problem?

  23. A misfire could also be caused by faulty valve cover gaskets. The spark plug wells have separate gaskets that can fail and leak oil into the spark plug well(s). Once the coil finds an easier way to ground it will take it. Also, your plugs will become fouled by the oil and fail to fire. Great content as usual. I just thought I’d add this as it is definitely a possible cause for a misfire.

  24. I have a 2015 honda accord. I bought my car new. In July of 2018 I ran over a tired piece on the freeway during rush hour and it did extensive damage to the tune of $7,000. I was not at fault. A new engine and several other things were put in. SInce then my car has been acting ‘weird” in that I will turn the car on and all the lights will go on – check engine, steering, etc and then I will turn it off and it will go away. One time these lights all came on and my car wouldn’t accelerate much. THis was in May of 2020. I brought the car to honda and they found an error in cylinder 3 (thats what the code indicated – and how I found your website. I read the article). They charged $500 but Im not sure what they actually did other than replace all 4 spark plugs and recommend that I use premium fuel for a few fills. This brings me to last week. I turned my car on, the same lights went on and my car once again had a difficult time accelerating. I brought it in and they now are charging $3,000 for “engine wiring hardness for ignition coils” and for ignition coil replacement with harness. They say in the notes that someone has attempted to repair coil 3. They asked for all my service orders (all service always done at hondas ) and realize the only service pertaining to cylinder 3/misfiring was done by them in May of 2020. I told this to a friend and he said this does not sound right given my cars age and mileage (100,000 miles). I do nothing special with my car (no accessories or off roading) I just use the freeway to get to and from work. In the notes from 2020 it looks like all they did was replacement and check spark sparks when I had a misfire in cylinder 3. Did they mess up? What should I do? I know nothing about cars but this seems off. Please help!

    1. They are screwing you over mama….u need plugs and spark plugs cables… Better to just replace both at same time.. just to let you know, really dough.

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