Ignition Systems ControlOBD2 Codes

P0304 – Meaning, Causes, Symptoms, & Fixes

P0304 cylinder 4 misfire detected

What Does Code P0304 Mean?

  • P0304 definition: Cylinder 4 Misfire Detected
  • Issue Severity: SEVERE. Do not keep driving with this error code.
  • Repair Urgency: Fix this code immediately (same-day if possible) to avoid serious engine damage and dangerous driving conditions.
  • Diagnosis: A multiple misfire can be caused by anything from faulty spark plugs to low engine compression. Because there are so many variables that could cause a misfire, the best cost savings is to find a trustworthy shop that can diagnose code P0304 as quickly and accurately as possible.

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Your vehicle moves when gasoline is burned and power is generated inside a chamber known as the cylinder. Most engines have a 4, 6, or 8-cylinder engine, where more cylinders typically mean more power. Power is generated by pistons that move up and down while fuel is ignited at very specific times. A misfire typically occurs when the timing of this ignition is off.  P0304 indicates that cylinder 4 is experiencing misfires.

P0304 Causes

Cylinder 4 misfires can be caused by many reasons from a faulty ignition system, fuel system, or internal engine failure. The most common reason for this to happen is faulty or worn-out spark plug coil packs, especially if it’s been a while since you had a tune-up.

  • 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

Code P0304 Symptoms

  • 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
  • Commonly associated with error code P0300

How Do I Fix Code P0304? 

With a cylinder 4 misfire fault, the first step is to get it diagnosed to figure out what is causing the engine to misfire. If your vehicle is misfiring and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal certified shop nearby to pinpoint the problem and give an accurate estimate for repairs. 

These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.

> Find a RepairPal Certified Shop Near You

How Much Does It Cost to Fix Code P0304?

P0304 can be caused by anything from old spark plugs to vacuum leaks to poor engine compression. It’s impossible to give an accurate estimate without properly diagnosing the issue first. 

If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your P0304 code.

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  • Upfront cost estimates
  • Minimum 12-month, 12,000-mile warranty
  • Fair price guarantee

Other Possible Repairs for Code P0304

Once properly diagnosed, P0304 may require one or more of the following repairs to resolve the underlying issue. These prices are based on national averages and include parts and labor. Your cost may differ depending on your location and type of vehicle.

  • Spark plugs: $66-$250
  • 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

DIY Steps to Diagnose Code P0304

Code P0304 can be caused by a number of underlying issues from spark plugs to compression to vacuum leaks. If you’d like to try to fix code P0304 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Keep in mind this is an intermediate-level diagnosis and repair and not recommended for beginners. Diagnosis requires more specialized equipment beyond what the FIXD Sensor can provide and it can be a time and labor-intensive process for inexperienced DIYers.

Difficulty of Diagnosis and Repair – Intermediate. This job requires some mechanical knowledge and is not recommended for beginners.

  1. Use FIXD to scan your vehicle to verify P0304 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 4). 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 4 coil and swapping it with the cylinder number 1 coil. If the misfire moved to cylinder number 1 (P0301), 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 4 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.

Code P0304 Common Diagnosis Mistakes

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

Still need help fixing code P0304?

If you’ve followed the steps above and are still experiencing misfires or check engine code P0304, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price.


Wife, mom, Content Manager & Senior Copywriter at FIXD. From the garage to the gym, I love helping people learn and grow. Dream car: ‘69 Acapulco Blue Mustang.

Kate McKnight
Wife, mom, Content Manager & Senior Copywriter at FIXD. From the garage to the gym, I love helping people learn and grow. Dream car: ‘69 Acapulco Blue Mustang.

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    1. Thanks! Good info!

      1. 2004 pt cruiser with number 1 misfire. Changed plugs and wires and still have a misfire at idle, only at idle. Runs perfect at normal highway speeds. What is going on, please help.

        1. mechanic said p304 302 misfire,and has low compression. car 2016 Hyundai sonata

          1. mechanic says p304&302 misfiring also low compression

        2. Mine was a relay in my glove box on the side I have a honda civic lx my car is now running good

          1. Started simple when FIXD said cyl #4, misfiring. Took out spark plug, huge gap, recapped, then checked, cleaned, reset all spark plugs. Now no misfire, runs smooth again. So my accord VSC warning light, detected, now out, a overgapped spark plugs……whew, dodged perhaps an expensive unneeded seRV ice charge !

    2. Thanks for all the good info. I just had this code pop up on my 2010 Forester i’m going to start with plugs and wires and inspection. your web site gave me a very logical set of steps to take to alleviate the code

      1. If it’s just water, check your hood seal/weather strip. Some of the older models had issues with water getting past the
        cowl seal between the windshield and hood. Usually that got in to the #4 plug well and caused issues, but it could
        run elsewhere along the top of the engine. I blew out the water with air and it solved all the issues. This was of course after changing Cam and Crank Sensor and VVT Sensor

    3. Better than average, 1st to suggest a cam or crank sensor, although my problem comes and goes and is lessened by using octane booster. I have swapped plug and coil packs with another cyl, still throws a p0304. When it runs good, it runs great, and Will as soon as you clear the code, then when your light comes on, it is very sluggish. I would think maybe cam timing actuators, but they have been replaced, and it runs too good when it does to suspect an exhaust restriction, leaking valve or vacuum leak. Maybe I will swap fuel injectors next.

      1. I’ll already change the sparkplug , ignition coil , air filter and cabin filter. after a month the engine lights on again the same problem as the previous reading p0304 . it’s like 4x that pop up after I clear the vode and check the cause and replace it again.. whats the next move to this kind of situation. can you help me pls.?

        1. Did you change your leads with the coil? that could be the problem

        2. Random misfires codes that come up normally are caused from your cam position sensor going bad or if it’s just po304 then it is your air fuel mixture sensor is open or a simple fact of you over filled your fuel tank

    4. Coil packs OK spark plugs ok
      Ecu ok
      Has been to the garage for two weeks. All electricals are working
      Can’t sort it out

    5. On older cars with 80k or more miles where I bought cheap gas, this code was caused by the spray pattern on the fuel injectors. I bought 32oz bottles of fuel injector cleaner and that got me 15k more miles down the road and the engine light turned off by itself. When it came back on a couple years later, I had the Toyota dealer test the injectors and they validated two were bad. Then I took to a local mechanic who replaced them for half price of the dealer. The $1800 in expense is far greater then the savings of buying cheap gas. Now I only buy “Top Tier” gas.

      1. My accord runs a whole lot better w shell gas. My car doesn’t love Exxon gas

    6. My car has the check engine one and it say cylinder misfire on number or and another cylinder misfire but I changed my spark plugs n it was good for 3-4 months than my check engine came on and ppl says it’s prolly a head gasket leak but idk

    7. Mine is a Seat Leon 2.0 tdi 170hp. It happens only over 80degrees engine temp. Until 80degrees its ok. Any ideea what can i check first?

    8. engine misfire when accelerating 2019

      1. I have a 2005 limited pt cruiser with turbo. I have replaced spark plugs, wires, ignition coil, and gas cap. I still seem to have this P0304 error code. I’ve tried Lucus injector cleaner when I gas up and still no luck. When I’m driving over 40mph, it seems to drive fine but when I am at stop lights or at idle it chugs and shakes really bad and hesitates like it wants to stall out. I cant seem to figure this out? Any ideas?

    9. I have a Ford KA Luxury 2007 and just wondered if someone could please help? I had error code P0304 showing but since Green Flag have replaced all four spark plugs I’m now getting P0303. They have tested all the electrics and confirmed they are fine. Therefore he suspects the issue is fuel related and suspects blockage in the petrol filter? He recommended I put some Redex in the fuel tank. To be honest the engine now seems to be misfiring worse than ever. The only other option he could offer was to tow me to the nearest garage. Does anyone know why the error code would change after replacing the spark plugs? Any help greatly appreciated. Thank you

    10. Can a mechanic give a low to high estimate? I mean depending on whether it’s a simple fix or a more complex problem for code 0304 or a cylinder 4 misfire.

    11. Enter your comment
      It is very helpful, Thanks!

    12. P0304. Changed plugs and ignition coil. Same code.

      Took injectors our and ran carb cleaner through them. Also swapped injector 1 and 4. Still P0304. Looks like the problem is not w/ the injector.

      At the end of the rope, going ask AAA for a tow to the shop. Hopefully they can figure it.

    13. if there is a misfire in cylinder #4 is that considered to be part of the engine as far as extended warrantys are concerned ???

    14. I have a 2016 Ford Fusion EcoBoost cylinder 4 misfire I change the plugs,coil packs,and fuel injector I don’t know what is causing this

      1. Caleb, I’m having the same issue now. What ended up being the problem?

      2. Hi Caleb, this is the same problem my freind has with his 2016 EcoBoost. The car delear said had a coolent leak into the #4 cylinder and now his engine needs to be replaced🤔
        What was your outcome with your problem?

    15. Mine was a relay in my glove box on the side and my distributor I have a honda civic lx my car is now running good

    16. Did you ever figure out the problem? I’m having the same issue with my 95 ford windstar.

    17. I have a 2003 Ford Explorer, and I had that code too and it turn out to be a bad head gasket. Water was going into cylinder 4 causing the misfire.
      Hopefully that helps someone.

    18. Hi Caleb, this is the same problem my freind has with his 2016 EcoBoost. The car delear said had a coolent leak into the #4 cylinder and now his engine needs to be replaced🤔
      What was your outcome with your problem?

    19. Just ran the code checker on a 2007 Lincoln MKZ and P0304 code came up. This is good information for repairs.

    20. What is typical cause for knocking accompanying this code. Reading cylinder 4 misfire worth knocking sound and loss of power. Also sputtering when accelerating. 2009 Silverado 6.0

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