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How do you fix the P0304 engine code?

Alton Alexander
By Alton AlexanderUpdated on June 4th, 2022

The P0304 code means that the engine is misfiring on cylinder 4. This is usually caused by a bad spark plug, ignition coil, or fuel injector.

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1. Check and clean spark plugs

  1. Remove the spark plugs from the engine.
  2. Clean all the insides of the plugs using a spark plug cleaner or a can of compressed air.
  3. Replace the spark plugs and reattach the wires.
  4. Start the engine and check for any misfires. If there are no misfires, the problem may be with the wires or the plugs. If there are misfires, the problem may be with the engine.

2. Check and clean fuel injectors

  1. Start the engine and let it idle.
  2. Check the fuel pressure.
  3. Check for leaks.
  4. If the leaks are not found, remove the fuel injectors.
  5. Clean the injectors using a solvent and a brush.
  6. Reinstall the fuel injectors.
  7. Check the fuel pressure and leak check.
  8. Start the engine and let it idle.

3. Check ignition system

  1. Verify that all of the electrical connections are tight and secure.
  2. Verify that the spark plugs are properly installed and firing.
  3. Verify that the air filter is clean and installed properly.
  4. Verify that the fuel injectors are properly functioning.
  5. Verify that the engine is properly lubricated.

4. Check engine compression

  1. Start your car and let it idle for a few minutes.
  2. Turn the ignition off and wait 10 seconds.
  3. Turn the ignition on and wait 10 seconds.
  4. Check the engine compression by pushing on the cylinder head with your hand. The compression should be between 12 and 14 psi. If it is not, proceed to step 5.
  5. If the compression is not between 12 and 14 psi, replace the engine air filter. If the compression is still not correct, replace the engine head.

5. Check and repair vacuum leaks

  1. First, it is important to identify the source of the leak. With vacuum leaks, the source can be anything from a loose connection to a cracked hose. Once the source of the leak is identified, the next step is to fix it.
  2. If the leak is small, it may be possible to fix it by tightening the connection or replacing the component. If the leak is more extensive, it may be necessary to replace the component.
  3. Once the source of the leak is identified and fixed, it is important to check for any remaining leaks. If there are any leaks, they will need to be repaired before the car can be driven.

Some users might also have success with the following opitons:

  1. Check and repair exhaust leaks
  2. Check and reset engine management system