P0304 Code

As engines accumulate miles over time, a P0304 code is one common yet serious indicator of an underlying problem in engine combustion. The P0304 is part of the Diagnostic Trouble Code (DTC) library under the extensive umbrella of the OBD-II codes, which serves to give car owners a glimpse of the inner workings of their vehicles. 

Learn more about the ins and outs of this error code from the pros at Hot Shot’s Secret, plus proper diagnostics methods, repair steps and tips for preventing code P0304.

Understanding the P0304 Code

A breakdown of the code is as follows: P (Powertrain) + 03 (Cylinder Misfire) + 04 (Specific cylinder affected). In simpler terms, a P0304 signifies “Cylinder 4 Misfire Detected,” which means that the powertrain control module (PCM) has sensed multiple misfires on cylinder four. 

Common Symptoms Associated With P0304

A P0304 code often comes with a series of signs, which may include:

  • Check engine light illumination or blinking
  • Vehicle stalling
  • Rough idling
  • Difficulty in uphill driving
  • Unpleasant odor from the exhaust
  • Excessive fuel consumption


Why Does the P0304 Code Occur?

A P0304 code indicates a repeating cylinder misfire, which occurs when one of the cylinders does not generate enough compression or spark. In a normal cylinder process, a given piston completes its compression stroke by working upward with the valves closed and compressing the air-fuel mix. Then, the ignition system delivers a high-voltage pulse through the spark plug from the coil. This voltage ignites the compressed gas, generating pressure and forcing the piston back down. 

Every piston plays a role in the total crankshaft load. Whenever the piston in a certain cylinder fails to do its responsibility, the operational activity of the crankshaft slows down, often referred to as “misfire.” As a result, when you start the vehicle or when the engine is idling, the cylinder remains unfired after incomplete combustion. There are several possible causes of a misfire in cylinder four, including:

  • Ignition system issues
  • Fuel system problems
  • Mechanical issues
  • Vacuum leaks


How to Diagnose a P0304 Code

While it can be tricky to diagnose a P0304 code because of the numerous possible causes, you can determine the main issue by utilizing OBD-II diagnostic tools. Follow the steps below and identify the underlying cause of the code:

  1. Locate the OBD-II port and scan the vehicle. If you come across other codes aside from P0304, make sure you fix them beforehand. After the initial diagnostic, perform a more detailed inspection of key parts. 
  2. Execute a visual inspection of relevant components. Checking the connectors and wires on cylinder four is crucial to uncover faulty connections. Tighten any loose wiring and replace damaged or frayed connectors. Make sure you also examine the ground wires. 
  3. Perform a cylinder balance test. Using a compression tester, check and compare the current readings of all cylinders. The compression test determines the measurement as the engine spins through its cycle. 
  4. Examine the spark plugs, ignition coils and fuel injectors. Faulty spark plugs, ignition coils and fuel injectors can cause a P0304 code. Swap each component with an individual part from another cylinder to assess the problem. If the issue transfers to the other cylinder, it means that specific component is faulty.
  5. Use advanced diagnostic techniques. On certain occasions, you may need an advanced scan tool to check the live engine data and read codes more accurately. A more detailed diagnostic system will make it easier to track misfires and view information related to fuel and intake. 


How to Fix the P0304 Code

Knowing the exact cause of a cylinder misfire before doing any engine work is crucial to prevent further damage to the vehicle and successfully clear the fault code. However, the repair process and the cost of fixing the P0304 code will vary depending on the nature of the issue, along with the vehicle’s make and model. 

Fixing the error code may require a series of repairs, such as: 

  • Replacing faulty spark plugs
  • Testing and replacing ignition coils
  • Examining and cleaning fuel injectors
  • Resolving internal mechanical issues
  • Fixing vacuum leaks


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