How To Fix P0304 Code For Honda Civic (Meaning, Causes, Symptoms, Diagnosis, Estimated Cost of Repair)

p0304 code honda civic

Do you know how to fix P0304 code for Honda Civic?  this is one of the questions our readers ask a lot. Well, we´ve got you covered.

Whenever your vehicle starts producing uneven exhaust sound, or the check engine light comes on, it’s a cause for concern.

Or at least a reason to pull out your very own OBD-II and check what is going on under the hood.

If you get the readout of the P0304 code then you should know that this is an issue you should deal with as soon as possible.

Not only that its symptoms are quite annoying and make driving your car quite rough, but it is also a situation that can easily escalate into a larger problem.

You are at the best place to find out everything you need to know about the P0304 code on Honda Civic and how to go about it.

Meaning of Code P0304 For Honda Civic

The diagnostic trouble code, or DTC, P0304 that you may see on your OBD-II scanner stands for “Cylinder 4 Misfire Detected”.

While an occasional cylinder misfire can happen for various reasons, code P0304 means that the fourth cylinder has misfired a sufficient number of times in a row for ECM to flag it as a problem.

This code means that there is a failure for combustion to occur inside the cylinder numbered as 4.

This can happen for several reasons, but the most common consequence is that the unburnt mixture of fuel and air is sent into the exhaust manifold and catalytic converter.

In turn, the fuel would ignite inside the exhaust system, which can increase the temperature of the catalytic converter well above its operational range, and eventually to a failure.

The potential failure of the catalytic converter is just one of the reasons why you should deal with code P0304 as soon as possible.

How to fix P0304 code for Honda Civic, Misfire Codes >> Check out the video below:

Causes of the Code P0304 On Honda Civic

There are several potential causes of code P0304 on the Honda Civic, some of them are quite easy to fix and will not break the bank, while some can be very expensive and complex issues.

So let’s cover them from the most to the least common ones.

The most common cause of a misfire is a bad spark plug.

The spark plug is supposed to generate the spark, if it goes bad or it is fouled it will not produce a spark.

But a bad spark plug can be a symptom of many other problems, as it goes bad for one of four causes, time, fuel, oil, or coolant.

It goes bad either because it is past its lifespan and the electrode is spent because for some reason there is too much fuel or not enough air in the cylinder, or there is some leak that lets oil or coolant into the cylinder.

Bad coil packs happen very rarely on Honda Civics, and in general, but when they do go bad P0304 or misfiring of any other cylinder is a likely consequence.

Symptoms of the Code P0304 On Honda Civic

Besides the check engine light, a sure sign of the misfiring cylinder is the rough sound of the engine.

But also the excessive vibrations, particularly at low RPMs.

If you continue driving your vehicle with code P0304, you will eventually notice a decreased milage.

Another characteristic symptom is the smell of gasoline from the tailpipe on a cold start.

Diagnosis of the Code P0304 On Honda Civic

Check the spark plug: To diagnose the most common potential problems, you need to take out the spark plug for the 4th cylinder.

If the spark plug porcelain and electrode are dark grey to matt black, for some reason that cylinder is running too rich a mixture.

A rich mixture is caused either because not enough air gets into the cylinder or because too much fuel does.

Potentially, the injector is stuck wide open, which you can check by placing a tip of a long screwdriver on the back of the injector while the engine is running.

If you place the ear on the handle you should hear the ticking, if you don’t hear it the injector is stuck open, otherwise, there is some vacuum leak.

If there are blotchy and greasy black deposits around the tip of the spark plug, the oil is leaking into the cylinder, most likely due to bad piston rings.

Gray or ash-like deposit on spark plug electrode is always due to coolant leak into the cylinder, which can happen due to a leaky head or intake gaskets.

Estimated Cost of Repair of the code P0304 On Honda Civic

Depending on the reason for the misfire the cost of the repair can go from $150 to $3,000.

When estimating the cost of repair, you should first take into account that the diagnosing entails an hour of labor.

So, here’s the overview of costs of some common causes, but keep in mind that the actual cost depends on many factors including the local labor rates:

  • Bad spark plug or wires – $100 to $300
  • Bad fuel system – $200 to $1,000+ depending on exact issues with fuel system
  • Faulty coil packs – $150 to $250
  • Bad injector – $275 to $400
  • Vacuum leak – $200 to $800

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Jeff is an automotive technician, technical writer, and Managing Editor. He has held a lifelong passion for cars, with a particular interest in cars like the Buick Reatta. Jeff has been creating written and video content about transportation, automotive, electric cars, future vehicles as well as new, used for more than 18 years. Jeff is based in Boulder, Colorado.

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