If you have scanned your engine computer to see what’s causing trouble and found the P0304 code, you might wonder what is going on and how to fix it. Thankfully, it doesn’t take a lot of troubleshooting to get to the bottom of this issue.
In this guide, we look at the meanings of the P0304 trouble code. We also show you what might happen when this occurs and evaluate how to repair it.
Table of Contents
Code P0304 Definition
P0304 – Cylinder #4 Misfire Detected
What Does the P0304 Code Mean?
P0304 DTC means that there’s an engine misfire in cylinder 4. The Powertrain Control Module (PCM) detects that a misfiring is occurring in the 4th cylinder in the car motor, thereby alerting the engine to set this trouble code. It’s a generic code, applying to all OBD-II equipped models.
While the P0304 trouble code might be considered generic, the repairs could depend on the make and model you drive. If there are multiple misfires occurring, you could also see the P0300 code together with other related codes indicating problems with other cylinders as well.
P0304 Trouble Code Symptoms
If one of the engine cylinders is misfiring, you should notice some performance issues. Additionally, the Check Engine Light will come on when the code is set, alerting you to the problem.
Here are some of the top symptoms you’ll notice with the P0304 DTC.
- Check Engine Light
- Rough idle
- Hard starts
- Jerking during idle or driving
- Loss of power during acceleration
- Lower fuel economy
Causes of the P0304 Code
When the car engine is misfiring, it can be caused by a number of problems. In some cases, the issues could stem from the ignition system, but it’s also possible that something has failed with the fuel system.
Here are some of the most common causes of the P0304 trouble code.
- Fouled spark plug
- Faulty Ignition Coil
- Defective coil pack/wires
- Bad fuel injector
- Leaking valve
- Vacuum leak
- Worn-out piston ring
- Leaking or blown head gasket
- Low compression
- Faulty computer module
How Serious is the P0304 Code?
Severe – The P0304 DTC is considered one that should be taken seriously. Driving with an engine misfire can become dangerous, especially if the car starts to stall in the middle of the road. If the engine stalls, it might be difficult to get it started back up, leaving you in a hazardous situation.
It’s important to have the P0304 trouble code examined as soon as possible. Running the engine with a misfire can also cause other engine troubles that will add to the repair bill. Plus, if the car breaks down, you might have to pay for a tow truck.
What Repairs Can Fix the P0304 Code?
You must perform a complete diagnosis before you attempt any repairs. Otherwise, you could spend unnecessary money on parts that aren’t needed.
Based on what fails most often, here are a few of the fixes that might be in order.
- Replace spark plug
- Replace ignition coil/wires
- Install a new fuel injector
- Fix leaking valve
- Fix vacuum leak
- Replace piston ring
- Repair leaking head gasket
- Replace computer module
Common P0304 Diagnosis Mistakes
Most commonly, the spark plugs, ignition coils, wires or coil pack is going to be to blame for the misfire. However, you should never assume that’s the problem without first performing the diagnostic steps we outline. You want to perform a visual check of the plugs, wires, coils, and coil pack before replacing it.
If these parts look okay, you need to move on to other parts that could be causing the trouble, such as the fuel injectors or valves. You also want to perform a compression test. Additionally, if there are any other trouble codes present, these might help you narrow down what’s going on.
How to Diagnose the P0304 Trouble Code
As a professional mechanic, you want to follow a laid-out plan when diagnosing the P0304 code. Gathering the code from the scanner is just the first step of figuring out what’s wrong. With the right steps, you will have a definitive answer to the problem.
Here are the most common diagnostic steps for the P0304 DTC.
- Collect the trouble codes with your OBD-II scanner and use freeze frame data to see what’s going on. Continue the troubleshooting based on the other trouble codes you may find.
- Reset the codes.
- Test drive the vehicle to see which codes come back.
- Inspect the spark plug, ignition coil, wire or coil pack on cylinder 4 to see if it is damaged or worn. Replace if necessary. You can try to move over the spark plug and ignition coil to another cylinder to see if you get a trouble code on the other cylinder instead – if so replace the faulty spark plugs and ignition coil.
- Check the intake system for a vacuum leak.
- If this isn’t the problem or the code doesn’t go away, you will need to inspect the fuel injectors.
- If you have an older car, you might also need to look at the distributor cap and the ignition cables.
- Beyond this, a compression test of cylinder 4 will be needed.
- Check the camshaft timing and inspect the timing belt or shaft if necessary.
- If nothing else can be found with the diagnostics, you might find the PCM is bad and needs to be replaced. However, with this problem, you’ll likely see other false codes or problems.
Estimated Cost of Repair
Depending on what’s causing the problem, you are looking at a varying amount of repair costs. Some issues are going to cost far less than others to fix. Below we list a few of the most common repairs and the average cost for parts and labor.
- Replace spark plug – $40-$350
- Replace ignition coils/wires – $75-$400
- Install a new fuel injector – $200-$500
- Fix leaking valve – $500-$2,500
- Replace piston ring – $1,000-$5,000
- Repair leaking head gasket – $1,500-$3,000
- Replace computer module – $750-$3,500
Mechanics Tips about the P0304 Code
If you find that the cylinder 4 spark plug is causing the p0304 code, you could simply replace that plug and move on. However, it’s always a better idea to replace all of the plugs at the same time. If one is failing, the others are probably shortly behind. Plus, by replacing the spark plugs all at once, you can stay on a recommended maintenance schedule with ease.
You also want to make sure that the spark plugs are adequately gapped. A gap that is too small or too wide can easily lead to a misfire. Use a spark plug gap tool to ensure the space matches the manufacturer’s specs.
Related Trouble Codes
- P0300 – Random or Multiple Misfire Detected
- P0301 – Cylinder #1 Misfire Detected
- P0302 – Cylinder #2 Misfire Detected
- P0303 – Cylinder #3 Misfire Detected
- P0305 – Cylinder #5 Misfire Detected
- P0306 – Cylinder #6 Misfire Detected
- P0307 – Cylinder #7 Misfire Detected
- P0308 – Cylinder #8 Misfire Detected
- P0309 – Cylinder #9 Misfire Detected
- P0310 – Cylinder #10 Misfire Detected
- P0311 – Cylinder #11 Misfire Detected
- P0312 – Cylinder #12 Misfire Detected
Frequently Asked Questions
Is it safe to drive with a P0304 code?
No, it is not safe to drive with a P0304 error code. The P0304 code indicates that your engine is misfiring on cylinder 4. If the cylinder is still misfiring, you risk damaging your catalytic converter or internal engine parts, which could lead to a very expensive repair.
How much does it cost to fix P0304?
To determine how much it will cost to fix the P0304 code, you need to diagnose and find the issue first. For example, if the P0304 code is caused by something simple as a bad ignition coil or spark plug, it could cost you between $50 and $150. If it is caused by low compression, it could in rare cases cost you over $5000 to fix the code.
How do I fix the error code P0304?
To fix the P0304 code, you need to diagnose and determine what is causing the misfires on cylinder 4. A bad ignition coil or spark plug is the most common cause of a P0304 code, and that is also where you should start the diagnosis.
What does the P0304 code mean?
The P0304 code means that the engine control module has detected a misfire in cylinder #4 that needs your attention. Misfires in a car engine can be caused by a failing spark, low compression, or fuel delivery problems.
What could cause the P0304 code?
There are many issues that can cause engine misfires on cylinder 4, resulting in a P0304 code. The most common cause of a P0304 trouble code is a faulty ignition coil or spark plug. Vacuum leaks or a faulty fuel injector are two other possible causes.