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  • The P0304 code stands for “Cylinder 4 Misfire Detected”, which means your vehicle’s cylinder 4 has misfired enough times for your powertrain control module (PCM) to notice.
  • A worn spark plug, a faulty fuel injector, and a leaking valve are some possible causes of the P0304 code.
  • Some common symptoms of the P0304 code are an illuminated check engine light, rough idling, and excessive fuel consumption.

The P0304 engine code can compromise your driving experience, so resolve the underlying cause (or causes) immediately. If your check engine light is on and the OBD-II scanner reads a P0304 code, use the guide below to address it.

What Does the P0304 Code Mean?

Diagnostic trouble code (DTC) P0304 stands for “Cylinder 4 Misfire Detected.” The code indicates that the powertrain control module (PCM) has counted enough misfire events on cylinder 4 to store the misfire code for that cylinder. Cylinder refers to the fourth cylinder as the cylinders are numbered.

checking cylinder head
Code P0304 indicates that the powertrain control module (PCM), has counted enough misfire events on cylinder 4 to store the misfire code for that cylinder.

Jump to the next section to quickly learn the most likely causes of code P0304. To learn more about firing order, read our explanation here. For an in-depth discussion of how the engine computer knows which cylinder is misfiring, you can read more here.

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Finally, for more information on how misfires happen, you can check out our technical explanation here.

P0304 on Some Ford Vehicles

The code appears in a 2009 Ford F-150 FX4 5.4L V8 because of the following conditions:

  • Air leaks in the Air Induction system (e.g., in the intake manifold)
  • Fuel delivery problem (e.g., a contaminated, dirty or sticking fuel injector)
  • Driving the vehicle low on fuel (less than 1/8 of a tank)
  • Base engine mechanical problem that affects only Cylinder 4
  • Air leaks in the EGR system and in its related hoses and/or valve
  • Air leaks in the PCV system and in its related hoses and/or valve

What are the Possible Causes of the P0304 Code?

There are a lot of components in your car’s engine and each one could prove to be the root cause of the trouble code. However, due to the trouble code being specific to cylinder 4, you can eliminate some areas and components that work irrespective of its functions. 

  • Ignition system problems, such as:
    • A faulty worn spark plug
    • A failed coil pack or coil-on-plug coil
    • A bad spark plug wire on a vehicle that has plug wires
  • Fuel delivery issues, such as:
    • A faulty fuel injector
    • Wiring issues with the fuel injector circuit (e.g., loose connections, damaged wires)
    • Problems with the powertrain control module, such as a damaged injector driver
  • Engine mechanical problems, such as:
    • A leaking valve
    • A worn piston ring, a damaged piston or damaged cylinder walls
    • A worn camshaft lobe or damaged lifter
    • A leaking head gasket
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mechanic holding a dirty and worn spark plug
A faulty worn spark plug can cause the P0304 code.

What are the Common Symptoms of the P0304 Code?

How can you tell if you’re dealing with a P0304? Even before you use an OBD-II scanner to check for trouble codes, there are a number of signs you can keep an eye out for that can alert you to the possibility of a P0304 or any similar error codes.

There are several P304 engine misfire symptoms to look out for. Below are some of them:

  • Check engine light is turned on
  • Difficulty in starting the engine
  • Stalling
  • Rough engine idling
  • Diminished power, especially when driving uphill
  • Excessive fuel consumption
  • Exhaust emits a foul odor

How to Diagnose the P0304 Code

P0304 must not be ignored. All potential causes must be looked into to come up with the correct diagnosis. Consider taking your vehicle to a professional automotive technician if you don’t have enough DIY auto repair experience and skills. However, if you still want to take a stab at the diagnosis process, the videos below can help you do it correctly.

How to Fix the P0304 Code

When it comes to OBD-II trouble codes, there is no such thing as a “magic bullet” solution. The repair process can vary depending on the root cause of the code, or even the vehicle’s make and model.

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For example, some of the most common documented fixes for a P0304 code on a 2008 Ford F-150 XL 4.2L involved replacing either an ignition coil, a spark plug, or a fuel injector. Meanwhile, the most common P0304 repair for certain Dodge vehicles, such as a 2014 Dodge Durango, Limited 3.6L, involved replacing the offending cylinder.

If you’re not confident with your automotive know-how, it may be best to leave the task to an expert. However, if you are certain of your DIY skills, identify the underlying issue that triggered the P0304 code. Then, consult online auto repair resources and guides to figure out the right solution to the problem.

You can also invest in an ALLDATA single-vehicle subscription, which can help you solve other automotive issues that you may encounter in the future.

Make sure to check your owner’s manual before attempting to fix a code P0304.

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About The Authors
Richard McCuistian, ASE Certified Master Automobile Technician
Reviewed By Richard McCuistian, ASE Certified Master Automobile Technician

Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

CarParts Research Team
Written By Research Team

Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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