Error P0304 – how to fix?

Code definition P0304

P0304 is a diagnostic trouble code that appears when an ignition fault is detected on cylinder 4. This code may be seen with other misfire codes and poor / bad codes.

What does the code P0304 mean

Code P0304 will be seen on cars where the ECU has detected that cylinder 4 is not firing properly. The ECU monitors all cylinders to ensure they all fire on time, so when one of them fails repeatedly, the Check Engine light comes on.

What causes the code P0304?

• Defective spark plug
• Faulty spark plug cables, spark plug and rotor and / or ignition coil pack
• Mechanical engine problem
• Inlet leak
• Fuel problems

What are the symptoms of the P0304 code?

• Engine runs poorly or has trouble keeping it running
• The engine light comes on
• Dud
• No engine power
• Reduced fuel consumption

How can a mechanic diagnose the P0304 code?

• The mechanic will begin by plugging a scan tool into the DLC port to determine what codes are present.
• Each code found is associated with frozen frame data that must be noted for use during the test drive.
• The codes will be cleared and then a road test will be carried out in conditions similar to those under which the previous data freeze was carried out.
• Then perform a visual inspection for damaged wiring, worn or damaged components, or visible leaks.
• Then use the scan tool to check the misfire counter for cylinder 4.
• The scan tool will then be used to view the long term fuel trim to see if fuel problems are present and the intake air pressure is checked for vacuum leaks.

The most common errors when diagnosing the code P0304

Errors can be made when components are not checked or unnecessary components are replaced. To avoid this, follow all the steps without skipping any. Replacing spark plugs and wires solves this problem on most vehicles. However, the entire system should be tested before determining what needs to be repaired.

How serious is the P0304 code?

• The P0304 code should not prevent the vehicle from being driven to a safe location when symptoms appear. Due to potential driving problems associated with this code, it is recommended that you repair the code as soon as possible to ensure safe operation of your vehicle.
• What repairs can fix the code P0304?
• Replacing spark plugs
• Replace cover and rotor, coil pack, spark plug wires (depending on vehicle)
• Repair of mechanical engine problems
• Repair air leaks
• Repair of fuel faults

Additional notes regarding code P0304

On vehicles with higher mileage coil packs, the coil pack may fail. An easy way to find out if the coil itself is to blame is to move the suspect coil pack to another cylinder. The vehicle should then be tested to see if the code changes to a new cylinder. If so, then the coil pack itself is damaged.

Depending on the year, vehicles will be fitted with coils that are state-of-the-art, spark plug wires that are a bit older, or a hub cap and distributor rotor that is usually found on carbureted vehicles. All of these items are considered wearing parts and are prone to failure on vehicles with higher mileage. Spark plug wires and cap and impeller are components that are typically replaced with spark plugs at routine intervals.

An advanced level scan tool is required for the correct diagnosis of P0304 – one that not only reads the code but gives you access to the current engine data. You will need this data to track misfires and view fuel and intake information.