P0304: Cylinder Number 4 Misfire Detected

What Is P0304?

P0304 is defined as “Cylinder 4 Misfire Detected”. 

It is an OBD-II Diagnostic Trouble Code (DTC) that indicates problems with your car’s cylinder number 4. P0304 may be seen with other misfire or lean/rich codes such as P0301 (Cylinder 1 Misfire Detected) or P0118 (Engine Coolant Temperature Circuit High Input.)

The Electronic Control Unit (ECU) monitors the operation of all cylinders to ensure they’re firing on time. Each cylinder is a power unit of your engine. It’s where the raw fuel (delivered by the fuel pump) is burned and then converted into mechanical energy to power the vehicle. 

A cylinder consists of a piston that oscillates for fuel compression, and a spark plug that ignites the fuel for combustion. It also contains two valves near the cylinder head — the inlet and outlet valves. Here’s what they do:

◾ The fuel and air enter the cylinder through the inlet valve from the fuel injector. 

◾The intake valve then closes to create a vacuum seal, causing the compression of the air-fuel mixture.

◾ That’s when the spark plug creates a tiny spark and ignites the mixture.

◾ Once power is generated, the piston pushes exhaust gases (bad gas) through the outlet valve.

But what causes the misfire that triggers the code?
A misfire is caused when one or more cylinders in your engine don’t produce enough power. In other words, it indicates that your engine’s combustion reaction isn’t working correctly. 

As such, a misfiring cylinder 4 will trigger the ECU to generate code P0304, and illuminate your check engine light.

Common Symptoms

The P0304 cylinder misfire code can result from ignition and fuel system issues. So, identifying its symptoms is essential to get to the root cause of the P0304 code. You may encounter symptoms like rough idle of the engine, and a few others, every time you get this trouble code. 

Here are some of those symptoms: 

1. Reduced Engine Performance 

A faulty ignition system can negatively affect engine performance and cause rough idle, engine noise, etc.

Several ignition components could cause these issues, including:

• Bad coil
• Faulty coil pack
• Fouled spark plug
• Damaged ignition wire 
• Broken cylinder head

As a result, you’ll experience reduced engine performance, including issues with fuel efficiency. 

2. Smell of Raw Fuel from the Exhaust

In this case, the smell of raw fuel indicates excess, unburnt fuel in cylinder 4. It could be due to an inoperable ignition coil or coil pack, a faulty fuel pump, or a worn fuel filter.

Whatever the case, it needs immediate attention to prevent a 4 cylinder misfire. 

3. Hesitant Acceleration

If your 4 cylinder misfire results from a faulty spark plug or damaged plug wire, your engine will experience a loss of engine speed. This occurrence happens due to incorrect combustion of the air-fuel mixture. 

You may need to get new spark plugs and change your spark plug wires to enable smooth acceleration and good engine speed (RPM).

4. Illuminated Check Engine Light

A lit check engine light is one of the most common symptoms of the P0304 code. However, your car’s engine light may also indicate p-codes pertaining to the ignition system, fuel pressure, engine coolant temperature, or other engine components.

So, it’s best to let your mechanic accurately identify the trouble code. They may perform a compression test to isolate the root cause of p-code P0304. The compression test also ensures that a mechanic works on the right cylinder. 

Can I Still Drive?

Error code P0304 is pretty severe and shouldn’t be ignored. Naturally, most mechanics will strongly advise against driving with a misfire code.

Why?
An engine misfire can slow your vehicle’s acceleration power and cause rough idling, leading to severe engine damage or accidents.

So if you do notice any of the symptoms of code P0304, contact your mechanic and get your engine checked to avoid costly repairs.

P0304 Causes

Several issues can trigger the P0304 code, but most pertain to a faulty ignition or fuel system. That said, error code P0304 can also spring from problems with the internal engine.

Below are common causes of error code P0304:

• Faulty spark plug
• Faulty spark plug wire (including chipped spark plug wire connector) 
• Faulty distributor cap, rotor
• A damaged ignition coil or coil pack (or loose ignition wire or cylinder head)
• Faulty fuel injector
• A vacuum leak, leaking intake gasket, or low fuel pressure
• Low engine compression
• Poor quality fuel
• Damaged O2 sensor
• Blown head gasket
• Worn engine valve cover
• Intake manifold gasket leak

P0304 Diagnosis

Diagnosing the P0304 cylinder trouble code allows targeted repairs and maintenance to restore optimal performance and prevent further damage.

Here’s what will happen during diagnosis:

1. Your mechanic will start by resetting the error code, taking a test drive, and checking to see if it persists.

2. If it does, they’ll visually inspect the spark plugs, spark plug wire connector, and ignition components, like the coil pack, or distributor cap, for any damage.

3. Your mechanic may also use the scan tool to monitor the misfire counter for cylinder 4 and collect freeze frame data.

4. Once the necessary repairs are made, your mechanic will check the intake air pressure to ensure no vacuum leak is present.

5. They may also use a fuel pressure gauge to monitor long-term fuel trims to see if fuel issues exist.

While error code P0304 indicates a misfiring cylinder, the root cause can be many. Your car’s ECM may trigger this trouble code due to a faulty fuel injector, a leaking intake manifold gasket, fouled spark plug, or low fuel pressure, among other things.

So it’s imperative to fully diagnose the main issue behind your misfire code before making the appropriate repairs.

Possible Repairs for P0304

P0304’s repair usually depends on the primary issue that triggered the engine misfire code. 

For instance, if a faulty fuel injector causes a misfiring cylinder, you’ll need a fuel injector replacement. Likewise, a worn ignition wire or a bad coil will warrant the replacement of ignition components.

These are the standard error code P0304 repairs that your mechanic may perform:

• Spark plug replacement 
• Fuel injector replacement 
• Fuel filter or fuel pump replacement
• Catalytic converter replacement (rare)
• Ignition components maintenance, etc.
• O2 sensor replacement
• Head gasket replacement