P0304 OBD-II Trouble Code: Cylinder 4 Misfire Detected

Check Engine Light

P0304 code definition

P0304 is a diagnostic trouble code seen when a misfire is detected in cylinder 4. This code may be seen with other misfire codes and lean / rich codes.

What the P0304 code means

The P0304 code will be seen in cars where the ECU has detected that cylinder 4 is not firing properly. The ECU monitors the operation of all cylinders to ensure they are all firing on time, so when one misfires repeatedly, a Check Engine Light is set.

What causes the P0304 code?

  • Faulty spark plug
  • Faulty plug wires, cap and rotor, and/or coil pack
  • Mechanical engine issue
  • Intake leak
  • Fuel issues

What are the symptoms of the P0304 code?

  • Engine running poorly or having trouble staying running
  • Check Engine Light present
  • Misfires
  • Lack of engine power
  • Decreased fuel mileage

How does a mechanic diagnose the P0304 code?

  • The mechanic will begin by hooking up a scan tool to the DLC port to determine which codes are present.

  • Freeze frame data will be associated with every code found and should be noted for use on the test drive.

  • The codes will now be cleared and a road test performed under similar conditions to the freeze frame data from earlier.

  • Next, a visual inspection will be performed looking for damaged wires, worn or broken components, or visible leaks.

  • Next, the scan tool will be used to monitor the misfire counter for cylinder 4.

  • Then, the scan tool will be used to look at the long-term fuel trims to see if fuel issues are present and the intake air pressure checked to see if vacuum leaks are present.

Common mistakes when diagnosing the P0304 code

Mistakes can be made when components are not tested or when unnecessary components are replaced. To avoid this, follow all steps without skipping any. Replacing the spark plugs and wires fixes this issue for most vehicles. However, the entire system must be tested before determining what to repair.

How serious is the P0304 code?

P0304 should not prevent the vehicle from being driven to a safe location once symptoms start. With the potential for drivability problems associated with this code, it is advised to have it repaired as soon as possible for safe vehicle operation.

What repairs can fix the P0304 code?

Additional comments for consideration regarding the P0304 code

Coil pack equipped vehicles with higher mileage may experience coil pack failure. An easy way to test if the coil itself is at fault is to move the suspect coil pack to another cylinder. Then test drive the vehicle to see if the code changes to the new cylinder. If it does, then the coil pack itself has failed.

Depending on year, vehicles will be equipped with either coil packs which are the most modern, spark plug wires which are a little bit older, or a distributor cap and rotor which is usually on carbureted vehicles. All of these are considered wear items, and are prone to failure on higher mileage vehicles. The spark plug wires and cap and rotor are items that are generally replaced along with spark plugs at routine intervals.

An advanced level scan tool will be needed to properly diagnose P0304 - one that does not just simply read the code, but gives access to live engine data. This data will be needed to track misfires and view both fuel and intake information.

Need help with a P0304 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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