How To Fix P0304 Code for Hyundai Sonata? (Meaning, Causes, Symptoms, Diagnosis)

P0304 Code Hyundai Sonata

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

A vehicle error code is never something you want popping up on your dashboard. While some codes are less severe than others, you should always have them checked and resolve the offending issue.

If your Hyundai Sonata has issued a P0304 code, you’ll want to get it checked immediately. While it’s a common code, it code threaten your vehicle’s drivability.

Code P0304 is a cylinder-specific misfire code, meaning that one of your Sonata’s cylinders is not firing in the correct order. This can cause some pretty obvious symptoms that you’ll quickly feel as you drive.

The problem, though, is not always directly caused by the cylinder so you’ll need to check out a few other parts as well.

What is a P0304 Code for a Hyundai Sonata?

Code P0304 indicates that Cylinder 4 in your Sonata’s engine is misfiring—either at the wrong time or not at all.

If this is the case, you’ll also notice some of the following symptoms along with OBDII Code P0304:

  • Your Sonata will issue a check engine line
  • Your vehicle may noticeably vibrate at lower RPMs
  • Your vehicle will generally not run as well
  • Your gas mileage will go down
  • You may smell raw fuel leaking from the exhaust pipe
  • Your engine may backfire on startup

If these symptoms are severe, immediately take your Sonata to be serviced.

Read also: How To Fix P0304 Code For Toyota Vehicles

Diagnosing a Code P0304 in a Hyundai Sonata

While Code P0304 indicates that Cylinder 4 is misfiring, it’s not always caused by the cylinder itself. Your engine may also have problems with the following issues:

  • Bad spark plugs – More often than not, bad spark plugs will trigger a Code P0304. If your spark plugs are old or worn out, your Cylinders are likely not getting enough energy to properly fire. Although most modern Sonata models come with long-lasting iridium plugs, they can still go bad and need replacing.
  • Bad spark plug wires – Hyundai Sonata spark plug wires might not be as long as what you’d find in older models but they can still go bad. If the wires are at fault, your cylinders will not get enough energy to fire correctly and your vehicle will issue a Code P0304
  • Bad fuel injector – Diagnosing a faulty fuel injector isn’t easy to do so we don’t recommend starting here but if your fuel injector has gone bad, your engine and cylinders won’t receive an adequate amount of properly atomized fuel. This will cause your cylinders to misfire and lead to a Code P0304
  • A leaky vacuum – If your Hyundai Sonata’s vacuum has sprung a leak, its engine won’t get a proper mix of air and fuel. This imbalance can cause your cylinders to misfire, leading to a Code P0304. This is especially true if the leak is near the cylinders themselves. This isn’t a difficult problem to diagnose but it’s not as common, so try checking your spark plugs first.
  • Bad crank or cam sensors – Bad cam or crank sensors are probably the least likely problem but they can sometimes lead to a Code P0304. If your ECU isn’t getting the right signals, it’ll mess up your cylinders’ timing, causing them to misfire.
  • Low Compression – Low compression can be caused by a whole host of other problems—a bent valve, cracked head, a leaky gasket, etc. If your engine doesn’t have enough compression, your cylinders won’t fire fully, causing your Hyundai Sonata to issue a Code P0304.

Hyundai Sonata Misfire – Diagnosis >> Check out the video below:

How Serious is a Code P0304?

If left unfixed, a Code P0304 can lead to some pretty dire consequences for your vehicle, making it nearly unusable.

It’s best to get it fixed immediately to prevent any long-term damage.

If the cause is something small, such as a bad wire or plug, you can expect to spend around $100-$200 for full service.

For larger problems, such as a leak or a bad injector, you could pay up to $1,000.

How to Fix a Code P0304 on a Hyundai Sonata

Before you can fix a Code P0304, first inspect your vehicle to find what’s causing Cylinder 4 to misfire.

Start by checking the smaller issues, such as your spark plugs and wires. You can test these problems by rotating your spark plugs.

If you get a different P030X error code, it means the faulty spark plug is affecting a different cylinder and should be replaced.

If you find that the problem is with your plugs or wires, replace them and then reset the trouble codes. If the code reinitiates, there’s likely a problem with your engine’s fuel injection system.

For more severe issues, we recommend taking your vehicle to a trained mechanic.


An OBDII P0304 error code indicates that Cylinder 4 in your Hyundai Sonata’s engine is misfiring. A misfiring cylinder is most commonly caused by a faulty spark plug or spark plug wire.

However, more severe problems can also cause the cylinders to misfire. Inspect your engine and make the required repairs immediately to avoid further complications that would cost more to fix.

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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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