P0304 – Meaning, Causes, Symptoms, & Fixes

Code P0304 Definition

Cylinder 4 misfire detected.

  • Commonly associated with codes: P0300

What Does P0304 Mean?

P0304 indicates that cylinder number 4 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 P0304 occurs it should be fixed immediately as long term driving with engine misfires could cause consequential damage to your engine.  Many times P0304 occurs when there are worn out spark plugs, spark plug wires, or a faulty ignition coil.

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

P0304 Causes (* = 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

Code P0304 Severity – Severe    

P0304 should be repaired immediately. Ignoring this error could result in ignition failure, catalytic converter damage, and unsafe/dangerous conditions while operating the vehicle.

Code P0304 Common Diagnosis Mistakes

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

Code P0304 Diagnosis Steps

Tools Needed to Diagnose:

How To Diagnose P0304

  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. 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, then you have determined that the coil pack is faulty and needs replacement. Replace spark plugs and spark plug wires/ignition coil packs if needed and recheck for misfires.
    1. How to identify a fouled spark plug
    2. How to test spark plug wires
    3. How to gap spark plugs
    4. How to replace spark plugs
    5. 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:
    1. 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.
      1. How to check fuel pressure
    2. 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.
      1. How to check fuel injectors by ear
      2. 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 leakdown test to see if there are any mechanical problems causing your misfire. Some common mechanical problems that cause misfire can be:
    1. Broken valve spring
    2. Broken piston ring
    3. Worn valve guides
    4. Burned valve
    5. Timing chain or belt skipped tooth and engine is off time.
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Comments 7

Susan DeVoe

July 15, 2018

Thanks! Good info!

Michael Bell

August 22, 2018

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


September 17, 2018

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.


September 18, 2018

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.?


November 20, 2018

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

Steve branfield

November 23, 2018

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

John Lehew

November 25, 2018

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.