P0304 Trouble Code: Meaning, Symptoms, Causes, & Fixes

The proper combustion of the air-fuel mixture is very important for the efficient working of the engine. When the air and fuel mixture doesn’t burn properly, your engine starts misfiring. When the vehicle’s PCM/ECM notices an engine misfire, it sets up a particular trouble code. The engine code P0304 represents a misfiring in cylinder 4. This article mainly explains the P0304 code meaning, symptoms, causes, and how to fix it.

P0304 Code Definition

P0304 code stands for “Cylinder 4 Misfire Detected.”

What Does the P0304 Code Mean?

The P0304 code indicates that your powertrain control module (PCM) has detected that cylinder 4 is misfiring.

P0304 Code

Vehicles run due to the combustion of the air-fuel mixture in the combustion chamber. This combustion process produces power that is used to operate the engine. A vehicle engine usually contains 4, 6, or 8 cylinders.

As you increase the number of cylinders, your engine power will be increased. A piston moves upward and downward inside the cylinder at a specific time. Misfires generally occur when this ignition timing is off.

Your PCM continuously records the working of all engine cylinders to ensure all cylinders are firing properly. Whenever any of these cylinders start misfiring, PCM triggers a code, and your check engine light starts flashing. The P0304 trouble code indicates that your vehicle engine cylinder number 4 is misfiring.

Your engine may start misfiring due to different reasons. Therefore, whenever your vehicle engine starts misfiring, you should travel your car to the nearest workshop to fix the main issue. You shouldn’t ignore misfiring because it may cause engine damage or complete failure of the engine.

Causes of P0304 Code

Your vehicle triggers the P0304 engine code due to one or more of the following causes:

  • Faulty crankshaft sensor
  • Bad camshaft sensor
  • Low fuel pressure
  • Bad spark plugs
  • Vacuum leaks
  • Damaged wiring of the spark plug
  • Bad ignition coils
  • Damaged fuel tank
  • Loose or damaged fuel cap
  • Faulty oil distributor
  • Head gasket leaks
  • Faulty fuel pressure sensor
  • Bad air-fuel ratio sensor
  • Faulty fuel injector 
  • Bad MAP sensor
  • Contaminated fuel
  • Low engine compression
  • A damaged lifter or worn camshaft lobe 

Symptoms of P0304 Code

When you drive your vehicle with the P0304 code, it produces one or more of the following causes:

  • Check Engine Light illumination
  • Rough idle
  • Poor engine performance
  • Poor fuel economy 
  • Fuel smell from the exhaust pipe
  • Limp mode
  • Hesitation or Stumbling 
  • Increased emissions 
  • Engine vibrations
  • Engine stalling
  • Hard starting conditions
  • Smoke from the exhaust pipe

Let’s discuss these symptoms of P0304 engine code in detail:

1) Check Engine Light

Illuminated Check Engine Light

The illumination of the check engine light is one of the most common symptoms of the P0304 engine code. The PCM or ECM of your vehicle continuously monitors the performance of your engine. When it monitors a misfire in cylinder 4, it triggers the check engine light.

2) Rough Idle

A misfiring engine may cause a rough idle. As we discussed above, your engine misfires due to poor combustion of air-fuel mixture. When your air-fuel mixture doesn’t burn properly, it doesn’t generate sufficient power to operate the engine efficiently and causes a rough idle.

3) Poor Engine Performance

Poor engine performance is also a clear symptom of engine misfire. When the air-fuel mixture in the combustion chamber doesn’t burn properly, you will observe a clear reduction in engine power. When your engine doesn’t generate sufficient power, your vehicle won’t be able to run at high speeds.

4) Poor Fuel Economy

Poor fuel economy

The engine misfire may also lead to poor fuel efficiency. When your engine misfires, it indicates that there is something wrong with the combustion of the air-fuel mixture. This poor combustion may lead to excessive consumption of fuel, reducing fuel efficiency.

5) Limp Mode

The limp mode is a safety feature that automatically turns on when something is wrong with your engine. When your engine misfires, the vehicle’s computer may activate the limp mode to prevent costly repairs.

However, limp mode may also be triggered due to many other reasons. Therefore, it is recommended to diagnose the system properly.

6) Hesitation or Stumbling

When you drive your vehicle with the P0304 engine code, you can observe hesitation as your engine tries to deliver consistent power. This symptom usually becomes clear as you push your accelerator pedal.

7) Increased Emissions

An increase in exhaust emissions is also one of the clear symptoms of the P0304 code. The proper combustion of the air-fuel mixture also plays a vital role in controlling the exhaust emissions. When your engine cylinder starts misfiring, it may lead to higher emissions of unburned nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC).

8) Engine Vibrations

Engine vibrations may also be caused due to engine cylinder misfire. These vibrations become clear at low speed or idle.

9) Engine Stalling

Engine Stalling, symptoms of DTC P0304

The engine stalling is also a clear symptom of an engine misfire. In case of engine stalling, your engine shuts off completely because of poor combustion of air-fuel mixture and power production.

10) Exhaust Odor or Smoke

A misfiring cylinder may also lead to an unpleasant smell coming from the exhaust pipe. When your engine misfires, it may force the unburned fuel to enter the exhaust system, which causes smoke or an unpleasant smell.

11) Hard Starting Conditions

Your engine won’t be able to generate sufficient power due to poor combustion of the fuel and air mixture. When the engine will not produce sufficient power, it will be hard to turn on the engine.

How to Diagnose the P0304 Code?

You need the following tools to diagnose or fix the P0304 code:

  • Screwdrivers
  • Digital Multimeter
  • Pliers
  • Spark plug wires
  • OBD-II scan tool
  • Spark plugs
  • Socket, ratchet, and extension
  • Compression tester
  • Fuel pressure gauge
  • Plug spanner

Follow the below-given steps to diagnose the P0304 code:

  • Use an OBD-II scan tool to check the presence of the P0304 code. Record all stored fault codes, along with all available freeze frame data. Clear the codes and make a test drive. If the code returns, then move forward for further inspection.
  • Inspect for loose connectors at the ignition coils.
  • Inspect the wires for damage (especially cylinder 4).
  • Inspect the loose engine ground wiring.
  • Visually inspect the spark plugs and spark plug wiring. If your car has individual packs other than the spark plug wires, first remove the coil from cylinder #4 and replace it with the coil of cylinder #2. Now check; if your engine misfiring is due to cylinder #2 and the P0302 code is triggered, it means the coil pack is defective, and you should replace it. The same test can be done for the spark plugs of cylinder #4, i.e., moving spark plugs from cylinder #4 to cylinder #1, and if the misfire is P0301, it means your spark plug is bad. If necessary, replace the spark plug and spark plug wiring and again check for the misfire.
  • If your ignition system is working efficiently, there may be an issue within the fuel system. For this, inspect the components of the fuel system, such as the fuel injector, fuel pump, fuel regulator, and fuel pressure sensor. You must also check the fuel level and fuel pressure. This is because low fuel pressure may lead the irregular misfires on multiple cylinders.
  • After the fuel system and ignition system inspection, perform a compression test to check the engine’s compression levels. Low compression may also lead to engine misfiring.

Common P0304 Code Diagnostic Mistakes

  • Not properly inspecting all the parts
  • Replacing the unnecessary components
  • Inability to confirm the presence of the P0304 code.
  • Clearing the code without first verifying the problem
  • Not clearing the PCM codes after fixing the codes

How serious is the P0304 Code?

DTC P0304 is usually considered a serious problem. You shouldn’t ignore this code and fix it as soon as possible. This is because spark-related misfires can allow unburned fuel to enter the exhaust pipe, where it can superheat. This superheated fuel has the ability to damage the catalytic converter, eventually leading to failure.

In any case, the root cause of the P0304 trouble code must be diagnosed and addressed quickly. This avoids unplanned downtime and unnecessary repair costs.

Repair Cost of P0304 Code

After diagnosing, the P0304 code may need one or more of the following repairs to fix the original problem.

Spark Plug replacement$60 to $260
Spark plug wiring repair$170 to $250
Fuel injector replacement$1400 to $2000
Ignition Coils$220 to $650
Fuel pump replacement$250 to $1,090

What repairs can fix the P0304 Code?

One or more of the following repairs can fix the P0304 code:

  • Replacing the bad spark plugs
  • Replacing the bad fuel injectors
  • Replacing the bad fuel pumps
  • Adding more fuel if needed
  • Replacing the damaged wires to cylinder number 4
  • Replacing the bad fuel pressure sensor
  • Replacing the bad fuel regulator
  • Repairing the fuel rail leaks
  • Fixing the vacuum leaks
  • Repairing the exhaust leaks
  • Replacing the bad O2 sensor
  • Repairing the damaged fuel tank
  • Replacing the faulty fuel cap
  • Fixing the issues related to the PCM
  • Replacing the faulty ignition coils

How To Prevent P0304 Code?

Follow the following preventive tips to prevent the P0304 engine code from triggering:

  • Regular maintenance of your vehicle
  • Use high-octane fuel
  • Change engine oil according to recommended intervals
  • Avoid overloading the engine
  • Keep your engine clean
  • Avoid aggressive driving
  • Regular maintenance of your ignition system
  • Regular maintenance of your vehicle’s fuel injection system
  • Address minor issues promptly
  • Ensure regular maintenance of your cooling system

FAQ Section

Can I drive with the P0304 Code?

P0304 doesn’t stop your car from being driven to a safe location once symptoms start. This code indicates a misfire. A misfire indicates the engine’s internal problems. If you continue driving with misfiring, it may badly damage your engine. Misfire also generates different drivability issues, such as poor fuel economy, poor acceleration, or a reduction in engine power. Therefore, you shouldn’t drive your vehicle with this code for a long time.

Can a misfire fix itself?

No, the engine misfire can’t fix itself. It indicates a serious issue with your engine. However, if you don’t fix it on time and continuously ignore it, it may cause the complete failure of the engine.

How do I fix a cylinder misfire?

  • Inspecting the fuel pressure and fuel level. Add more fuel if the fuel level is less than a specific level.
  • Replace the fuel pump.
  • Replace the fuel injector.
  • Replace the fuel filter.
  • Replace the fuel pressure sensor.
  • Replace the spark plug.

Can a misfire damage your engine?

If left untreated, a cylinder misfire can lead to significant engine damage. Worse, if you experience a bad misfire while driving, it could result in an accident. This is why it’s important to treat engine misfires as soon as you detect them.

What is the symptom of error code P0304?

  1. Check Engine Light illumination
  2. Jerking/Hesitations when driving
  3. Fuel smell from the exhaust pipe
  4. Misfires
  5. A reduction in the engine power
  6. Poor fuel economy
  7. Rough idle
  8. Engine stalling
Read More
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  2. P0301 Code Symptoms and Causes
  3. P0302 Code Symptoms and Causes
  4. P0303 Code Symptoms and Causes
  5. P0305 Code Symptoms and Causes
  6. P0306 Code Symptoms and Causes
  7. P0307 Code Symptoms and Causes

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