P0304 is a diagnostic trouble code (DTC) for “Cylinder 4 Misfire Detected”. This can happen for several reasons and a mechanic needs to diagnose the specific cause for this code being triggered in your situation.
P0304 Code Definition
P0304 is a generic 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 does the P0304 code mean?
The P0304 code will be visible 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
- Mechanical engine problem
- Intake leak
- Fuel problems
What are the symptoms of the P0304 code?
- Engine runs rough or has trouble staying running
- Check Engine light present
- Misfires
- Lack of engine power
- Reduced fuel consumption
How does a mechanic diagnose the P0304 code?
The mechanic will start by plugging a scan tool into the DLC port to determine which codes are present.
Freeze frame data will be associated with each code found and should be noted for use on the test drive.
The codes will now be cleared and a road test will be performed under conditions similar to the previous freeze frame data.
Next, a visual inspection will be performed looking for damaged wires, worn or broken components, or visible leaks.
Then, the scan tool will be used to monitor the misfire counter for cylinder 4.
Next, the scan tool will be used to examine the long term fuel trims to see if fuel problems are present and the intake air pressure checked to see if there are any vacuum leaks.
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 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 begin. Given the potential for driving problems associated with this code, it is advisable to have it repaired as soon as possible for safe vehicle operation.
What repairs can fix the P0304 code?
- Replacing spark plugs
- Replacing the cap and rotor, coil pack, and plug wires (depending on the vehicle)
- Repairing mechanical engine problems
- Fixing air leaks
- Repairing fuel supply faults
Additional comments to consider regarding the P0304 code
Vehicles equipped with coils with higher mileage may experience a coil pack failure. An easy way to test if the coil itself is at fault is to move the suspected coil pack to another cylinder. Then test the vehicle to see if the code changes to the new cylinder. If it does, the coil pack itself is faulty.
Depending on the year, vehicles will be equipped with either coils which are the most modern, slightly older spark plug wires, or a distributor cap and rotor which are typically found on carbureted vehicles. All of these items are considered wear items and are prone to failure on higher mileage vehicles. Spark plug wires, cap, and rotor are items that are typically replaced with spark plugs at regular intervals.
An advanced level scan tool will be needed to properly diagnose P0304 – a tool that doesn’t just read the code, but gives access to live engine data. This data will be needed to track misfires and display fuel and consumption information.