P0304 – Cylinder 4 misfire detected

The cylinders in your engine are what provide its power. When they don’t fire correctly, your car will drive rough and be at risk for further damage. The P0304 OBD2 code is your first warning sign of misfires in your engine.

The P0304 code can be severe, so it’s not one you want to ignore. Read on below to learn more about what it means and how you can correctly diagnose it.

P0304 code definition

P0304 Code Definition (Generic): Cylinder 4 misfire detected

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Here's where you can get a thorough understanding of the P0304 OBD2 code
If you want to learn about the P0304 OBD2 code, this is the right place

What does P0304 mean?

The cylinders provide the power in your engines. This is where the spark plugs ignite the air/fuel mixture, generating the energy that turns the crankshaft. A misfire occurs when there isn’t enough fuel burning in a cylinder.

When the engine functions correctly, the cylinders fire in sequence to provide continuous power to the crankshaft. If the RPM of the crankshaft changes by more than 2%, this is when the engine will trigger the P0304 code. 

The P0304 OBD2 code explicitly indicates a misfire in cylinder 4. You can use your car’s manual to locate this cylinder. It is common to also see P0300 and other misfire-related codes as well with P0304.

What are the symptoms of the P0304 code?

You will often experience serious drivability problems when the P0304 trouble code activates. These include:

  • The check engine light activates. If the light is solid, the change in the crankshaft RPMs is between 2% and 10%. A flashing or blinking light indicates a variance over 10%. 
  • The engine runs rough. The engine may shake while idling. You may also notice jerking or hesitation from the engine while accelerating.
  • There’s difficulty starting or failure to start.
  • There’s a smell of fuel in the exhaust.
  • Engine power reduces.
  • Fuel economy reduces.

What are the causes of P0304?

The most common causes of the P0304 trouble code are:

  • Faulty or worn-out spark plugs
  • Faulty wires around the spark plugs
  • Faulty spark plug coils
  • Failure of the distributor
  • Faulty fuel injector

Engine misfires can also be a result of:

  • Leaks in the vacuum system
  • Fuel pressure is too low
  • Timing issues in the engine
  • Defective sensors (camshaft, crankshaft, oxygen, MAF, etc.)
  • Leaking head gasket
  • Fuel is too low quality for the engine 
 P0304 code mean that cylinder misfire
A misfire occurs when there isn’t enough fuel burning in a cylinder.

How serious is the P0304 code?

The P0304 OBD2 code is severe. Repeated misfires can damage your catalytic converter, ignition system, and other parts of your engine. The erratic performance also makes your vehicle a potential safety hazard. Stop driving your car when this code triggers, and refrain from driving until you’ve repaired the problem.

How to diagnose the P0304 code

Tools you’ll need:

  1. Scan your system for other OBD2 trouble codes. Fix any that come up other than P0300-P0308. 
  2. Inspect the wires around the spark plugs for damage and loose connections. 
  3. If your engine has individual coil packs, switch the cylinder 4 pack with the one in cylinder 1. Clear the codes and test drive your car. If the misfire jumps to cylinder 1, the pack is faulty and should be replaced. 
  4. Visually inspect the spark plug for carbon build-up or other signs of fouling. It’s easiest to see if you have a new spark plug for comparison. Remove and clean the spark plug. If the build-up can’t be removed, replace the spark plug. A spark plug that’s shiny or blackened likely indicates the fuel mix is too rich.
  5. Use a multimeter to measure the resistance of the spark plugs. Replace the wires if the reading is higher than 15,000 ohms. 
  6. Check the ignition coils. Remove the coil and touch the multimeter leads to both the primary and secondary ignition circuit. The primary ignition circuit typically reads between .4 and 2 ohms, and the secondary circuit between 6,000 and 10,000 ohms. Your vehicle’s manual can give you a more exact specification. If you get a 0 reading, the coil has an internal short.
  7. Use a fuel pressure gauge to test the fuel pressure. Compare the reading to the specifications in your vehicle’s manual. 
  8. Unplug the fuel injector from your engine and probe the terminals with a multimeter. Compare the resistance reading to the specifications in your manual. A variance could indicate a faulty fuel injector. 
  9. Perform both an engine compression test and a leak down test. Mechanical problems like worn valve guides, burned valves, skips in the timing chain, and broken valve springs or pistol rings can lead to misfires. These tests will identify any failing components. 

Common mistakes to avoid while diagnosing the P0304 code

Ensure you don’t overlook small issues, like loose connections and vacuum leaks. Be thorough in your engine inspection to make sure you’ve caught every potential source of the misfire. 

What should you do to fix the code P0304?

If you’re not experiencing any symptoms, clear the code and test drive your vehicle. A single misfire may not indicate long-term problems. If the code does come back, continue with the steps below.

  1. Replace any damaged wires found in your inspection, as well as any other components that failed diagnosis. Ensure all newly-installed items are functioning and all connections are secure.
  2. Check the EGR system and fuel injectors for clogs. Clean out any that you find. 
  3. Test the sensors in your fuel and exhaust systems, including the crankshaft sensor, camshaft sensor, mass airflow sensor, and oxygen sensors. Inspect them visually for dirt or damage, then use a multimeter to ensure they’re receiving a charge. Replace any that are damaged. 

Tips to avoid P0304 in the future

Issues with your air-to-fuel ratio may cause misfires. Check all the hoses in your system for leaks as part of your regular preventative maintenance. Air that gets into your system can make your air/fuel mix too lean. Alternatively, oil or carbon build-up on the spark plugs indicates a mix that’s too rich. 

Using the wrong fuel can also cause engine misfires. More expensive fuel isn’t always the answer. Only use the grade recommended for your engine in your vehicle’s manual.

P0304 Error: Cylinder 4 misfire detected and replacing the old coil.