P0304 Code: What It Is And How To Fix It (2024)

Elantra

Make

Hyundai

Segment

Sedan

The internal combustion engine is truly an amazing technological marvel. Using multiple systems, computers, and components, combustion is the process of burning a precise mixture of air and fuel at the correct time. When the combustion process is interrupted, we call it a misfire. If your vehicle sets the P0304 Cylinder 4 Misfire Detected error code, your engine is experiencing misfires. Since most cars have at least four cylinders, it's possible that your car could be experiencing this error code if you notice an illuminated engine light and other signs of reduced performance.

What Does The P0304 Error Code Mean?

The P0304 error code indicates the presence of a misfire. The P0304 code specifically indicates the misfire is occurring on cylinder 4, as indicated by the last digit in the code; P0302 would refer to a misfire on cylinder 2, for instance. On most four-cylinder engines, the fourth cylinder is located closest to the driver's side of the vehicle. Misfires can cause serious internal engine damage due to the disturbance of the crankshaft's rotational speed. The Powertrain Control Module (PCM) monitors the speed of the crankshaft through a Crankshaft Position Sensor (CKP). If the PCM does not pick up the correct rotational speed from the CKP, a misfire is detected, and the error code is set.

What Causes The P0304 Error Code?

In the case of misfire-related error codes, there are typically several causes. The most common causes are worn spark plugs and faulty ignition coils.

The many elements of the combustion process, including the compression and combustion of an ideal air-fuel mix, should not be disrupted. If that's the case, it can lead to a misfire and compromise performance.

Potential causes for misfires are:

• Open or short in engine wire harness
• Damaged vacuum hose connections*
• Worn spark plugs
• Oily spark plugs (leaking plug seals)
• Faulty spark plug wire
• Leaking head gasket
• Corroded distributor cap contact
• Faulty ignition coil (coil-on-plug design)
• Fuel injector assembly (leaking or stuck ON)
• Incorrect fuel pressure (too low or too high)
• Dirty or malfunctioning mass air flow meter*
• Faulty engine coolant temperature sensor*
• Low cylinder compression
• Incorrect valve clearance
• Incorrect valve timing*
• Malfunctioning variable valve timing (oil and/or filter condition)
• Positive Crankcase Ventilation (PCV) valve and hose*
• PCV hose connections*
• Leaking intake system*
• Faulty Engine Control Module (ECM)*

*Will most likely set additional DTCs

Symptoms Of The P0304 Error Code

You can usually tell your engine has a misfire due to noticeable symptoms such as a flashing check engine light or poor acceleration.

The following are reported symptoms from an active P0304 fault code:

• Permanently illuminated or flashing engine light
• Failed emissions test
• The engine runs rough
• Reduced engine power
• Shaking or stumbling under acceleration
• Fuel odor smell
• Extended cranking
• Rough or erratic idle
• Reduced gas mileage

Can I Drive My Car With This Issue?

Due to the possibility of severe engine and catalytic converter damage, the P0304 error code should be remedied immediately. When a misfire occurs and the check engine light flashes, it indicates raw/unburned fuel is entering the catalytic converter. This raw fuel will break down the converters and cause them to clog. Catalytic converters use precious metals such as platinum, rhodium, and palladium, making them costly to replace. This is a cost you'll want to avoid.

How To Fix A P0304 Error Code On Your Vehicle

Before attempting to repair the P0304 error code, you need to verify the code is currently active. To do this, you'll need to have your On-Board Diagnostic (OBD-II) system scanned with a scanning tool. Once the code is confirmed, you may begin your diagnosis.

The P0304 error code can typically be repaired with a simple coil or spark plug replacement. In our article on the P0302 error code - a misfire on cylinder 2 - we also looked at how to perform a power balance test and inspect items like the spark plugs, coil packs, and fuel injectors. These are all possible remedies that could apply to the P0304 error code, too.

A common repair to remedy the P0304 error code is the replacement of the Positive Crankcase Ventilation (PCV) valve. The PCV valve is responsible for approximately 10% of the air entering the engine. If it becomes clogged or stuck open, it can cause a misfire.

Inspecting And Replacing The PCV Valve

Most PCV valves are mounted on the valve cover or cylinder head cover of the engine. Always reference the vehicle service manual for your PCV valve's exact location. Some of the tools required to inspect or replace a PCV valve are:

• A basic set of hand tools with a ratchet, assorted wrenches, and sockets
• Pliers set
• Screwdriver set
• Service manual

Inspecting a PCV valve involves removing the valve for closer inspection and inspecting the connecting hose's condition. The operation can be affected if any of the hoses connecting to the PCV are cracked or damaged.

Remove the PCV valve and shake it. If a metallic rattling sound is heard, the valve is okay. If the sound is dead or no sound is heard, the valve should be replaced.

How Much Does It Cost To Fix?

Considering the majority of reported P0304 error codes have been fixed by spark plug or coil replacement, the expected cost to resolve the issue should come in at around $100 - $300. RepairPal estimates ignition coil replacement between $200 and $300. PCV valve replacements will cost somewhere between $50 and $110.

Because your vehicle can still be driven, it may be tempting to ignore the P0304 error code. However, prolonged operation without repair could result in severe engine damage.