Estimates Trouble Codes P0304

P0304: Cylinder Number 4 Misfire Detected

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What is P0304?

Trouble code P0304 is defined as “Cylinder 4 Misfire Detected.It’s a generic powertrain code that may be seen with other misfire or lean/rich codes.

P0304 is an OBD-II Diagnostic Trouble Code (DTC) that indicates problems with your car’s cylinder number 4.

Since the Electronic Control Unit (ECU) monitors the operation of all cylinders to ensure they’re firing on time, a misfiring cylinder triggers the ECU to generate code P0304. This can manifest itself in many ways, including an illuminated check engine light or rough idle of the engine.

Your car’s cylinder is the power unit of your engine. This is where the raw fuel from the fuel pump is burned and then converted into mechanical energy to power the vehicle. The cylinder consists of a piston that oscillates for fuel compression and a spark plug ignites the fuel for combustion. It also contains two valves near the cylinder head — the inlet valve and the outlet valve. 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. This is when the spark plug creates a small spark and ignites the mixture.

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

A misfire is caused when one or more cylinders in your engine don’t produce enough power. In other words, it’s an indicator that your engine’s combustion reaction isn’t working correctly.

An engine misfire can be fairly serious and shouldn’t be ignored. It can slow down the acceleration power and cause rough idle.

Common symptoms

The P0304 cylinder misfire code can result from a combination of ignition system issues. So, it’s essential to identify its symptoms to get to the root cause of this p-code.

You may encounter generic trouble code symptoms like the check engine light, along with a few others, every time you get this trouble code. 

Here are some of those symptoms: 

1. Reduced Engine Performance 

A faulty ignition system (including a bad coil, coil pack, a damaged ignition wire, or cylinder head) can negatively affect engine performance and cause rough idle, engine noise, etc.

As a result, you will 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 the 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 cylinder 4 misfire. 

3. Hesitant Acceleration

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

You may need to get a new plug and have your spark plug wires changed.

4. Illuminated Check Engine Light

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

So it’s best to let your mechanic handle the troubleshooting using professional tools that will accurately identify the trouble code. They may also do a compression test to isolate the root cause of p-code P0304. 

Can I still drive?

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

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 trouble code P0304, but most of them pertain to a faulty ignition system or fuel system. That being said, error code P0304 can also spring from problems with the internal engine.

Here are some common causes of error code P0304:

  • Faulty spark plug
  • Faulty spark plug wire (including chipped spark plug wire connectors) 
  • Faulty 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, etc.


Most DTC codes can be easily diagnosed using an OBD-II scanner. Your mechanic will start by resetting the error code, taking a test drive, and checking to see if it persists.

If it does, they will visually inspect the spark plugs and ignition components for any damage, leaks, or other issues. Your mechanic may also use the scan tool to monitor the misfire counter for cylinder 4 and collect freeze frame data.

Once the necessary repairs are made, your mechanic will check the intake air pressure to make sure there’s no vacuum leak present. They may also use a fuel pressure gauge to monitor long-term fuel trims to see if fuel issues are present.

When diagnosing a P0304 cylinder trouble code, people may replace engine components without thoroughly testing all potential causes.

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 gasket, 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 misfire code. 

For instance, if a faulty fuel injector causes the 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.

That being said, here are the common 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.

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