Error Code P0304 is described as Cylinder #4 Misfire Directed, which means the vehicle’s computer has detected an engine misfire, specifically with the fourth cylinder.


Error Code P0304 is a common error code which signifies a misfire issue with the engine’s cylinder, particularly the cylinder #4. All modern vehicle models are now equipped with a computer and come with a misfire detection monitor that tells the scan tool about the issue, and helps pinpoint exactly which engine cylinder is having a hard time working properly.

An engine misfire means lack of combustion, which can be a result of many things, like bad fuel quality, low compression, lack of spark, or unmetered air entering the entering the engine. When a misfire occurs in the engine, it will automatically set the check engine light and show a number. The last digit of the number signifies the specific cylinder at fault. For example, if it’s cylinder #1 that is having trouble firing, the Error Code will be P0301. For cylinder #3, it will be Error Code P0302, and so on.

Common Symptoms

The common symptoms of this error code include:

  • Check Engine light is on
  • Rough riding, hesitation and engine jerking during acceleration
  • Engine dying at stop signs
  • Shortage of power
  • Bad fuel economy

Possible Causes

The common causes the triggers this error code includes:

  • Defective spark plug, distributor caps, ignition wire, and rotor (if applicable)
  • Defective O2 (oxygen) sensor
  • Defective MAF (Mass Airflow Sensor)
  • Defective throttle position sensor
  • Defective camshaft sensor and/or crankshaft
  • Defective coil (pack)
  • Defective fuel injector
  • Defective catalytic converter(s)
  • Bad fuel quality and running out of fuel
  • Low fuel pressure
  • Faulty computer and malfunctioning EGR system
  • Mechanical engine problem such as leaking head gasket, low compression or valve problem (burned exhaust valve)

How to Check

To start your diagnosis, check the ignition system operation and make sure there’s adequate fuel. Then, follow up with other causes such as EGR and EVAP system issues.

When diagnosing your vehicle for this code, it is important that you take note of the freeze frame information and duplication all the conditions that set the code on a test drive. This means you need to remember the engine load, throttle position, RPM and of course, road speed.

For a cylinder misfire problem, you need to change components, such as coils, spark plugs, etc. to isolate the root cause of the misfire. Also, make sure you jot down the other codes, as misfire can be caused by a malfunctioning component or system.

Here are the four different types of misfire and how to diagnose them:

Ignition Misfire

Look for signs of heat damage and wear and tear in every individual component of the ignition system. This includes the spark plug terminals, which should maintain a sandy color. If it looks blackened with soot, then it may because the combustion chamber is overheating. If it looks greenish, then there may be a problem with the coolant.

For the ignition coils and cables, there should never be arcing. Check the ignition system and make sure the firing voltages fires evenly, which should be 8V to 10V per cylinder.

Remove the distributor cup and rotor (if applicable) from the engine. Inspect the terminals and all contact points for wear and tear, corrosion buildup or arching.

Though most vehicles with OBD II comes with computer controller trimming, make sure you it has the right spec, even if it is using individual coils.

Lean Misfire

Take note of the Long Fuel Trim values as they identify how the PCM (Powertrain Computer) work to compensate and maintain the balance of air/fuel ratio. If there’s more than 10% in one of the cylinder bank and not on the other, then that means there’s a vacuum leak, defect or crack on the bank’s intake manifold. You will know what causes the PCM to do this compensation by checking the Fuel Trim “numbers” within the full range of operating conditions. Basically, a problematic engine has around 1% to 3% Long Term Fuel Trim numbers, either positive or negative.

Mechanical Misfire

There are two easy ways to diagnose a mechanical misfire: A Compression Test and an Engine Idle Manifold Vacuum Test. Steady compression readings (10% each) and a minimum of 120 PSI per cylinder, at least 17 inches steady vacuum are needed for complete and smooth combustion.

Powertrain Misfire

Badly warped and out-of-round rear brakes will cause an engine misfire. Symptoms of which involve violent PCM jerking and slowing down on highway speeds. To diagnose this misfire, have your vehicle’s engine checked thoroughly, and fix or replace any incorrectly perceived mechanical misfire that starts in the transmission, transfer case, driveshaft or front/rear differential.

How to Fix

For cars that don’t always show symptoms, the easiest way get rid of this code is to reset the code, drive, and test if it comes back again. If symptoms persist however, here are quick few fixes:

If the engine hesitates or stumbles, then you need to inspect the wiring connectors the connectors to the leads of the cylinder, particularly the spark plugs.

Replace any damaged ignition components during your scheduled regular maintenance. Normally, it is the distributor cap, park plugs, wires and rotor (if applicable) that needs replacement to fix this error code. If all seems to be in good shape, check the coil (coil packs).

If you sense the smell of rotten eggs in the exhaust, then that signifies a damaged catalytic converter, and you need to replace your CAT converter.

Faulty fuel injectors can also cause this error code.

Parting Tips

For more information about this error code and other cylinder misfire error codes, feel free to browse through our website. For professional help, Contact Us.