P0304 Code | The Guide to Diagnosis and Fast Fixes

Enter the error code and car brand

Vehicle engine and/or automatic transmission

Code: P0304 - The engine’s cylinder 4 is not firing properly (misfire detected), which can cause rough running and reduced performance.

When you see P0304 pop up, what your car’s telling you is that cylinder number 4 isn’t firing the way it should. In my experience, that means the air-fuel mixture in that cylinder isn’t igniting properly, and the engine’s going to feel it-sometimes in a big way. The powertrain control module (PCM) is constantly keeping tabs on each cylinder, counting every time combustion doesn’t happen like it’s supposed to. If cylinder 4 misses the mark too many times, the PCM flags it with this code. Basically, it’s your car’s way of waving a red flag, letting you know there’s a hiccup in that specific cylinder. Ignore it, and you could be looking at bigger headaches down the road-both for your engine and for emissions.

Causes of P0304

From experience, I can tell you that p0304 usually comes down to a handful of common culprits. Most often the issue is with the ignition system-think worn-out spark plugs, a failing ignition coil, or a bad spark plug wire if your vehicle uses them. But don’t rule out fuel delivery problems, like a faulty or dirty fuel injector, or even wiring issues in the injector circuit. Sometimes, it’s a mechanical problem inside the engine itself, such as a leaking valve, worn piston rings, or even a damaged camshaft lobe. Here’s a quick rundown of what I see most often:

Worn or fouled spark plug in cylinder 4
Failed ignition coil or coil-on-plug for cylinder 4
Bad spark plug wire (if equipped)
Faulty or sticking fuel injector for cylinder 4
Wiring issues in the fuel injector circuit
Leaking valve or head gasket in cylinder 4
Internal engine wear (piston rings, cylinder wall, camshaft lobe, lifter)
Air leaks in the intake, EGR, or PCV systems affecting cylinder 4

It’s usually best to start with the simple stuff before digging into the engine internals.

Symptoms of P0304 Engine Code

If you’re dealing with a dtc p0304, you’ll likely notice a few things right away. The check engine light will come on, and you might feel the engine running rough, especially at idle. Sometimes the car will hesitate or stumble when you try to accelerate, and you might even have trouble starting it. In some cases, you’ll notice the engine stalling, or you’ll smell a strong, unpleasant odor from the exhaust. Another thing to watch for is increased fuel consumption-your car might start guzzling more gas than usual. If you’re driving up a hill or putting the engine under load, you’ll probably feel a lack of power, too.

Diagnosis Steps for DTC P0304

Here’s how I usually tackle dtc p0304 ford and dtc p0304 toyota. First, I recommend starting with the basics-pop the hood and check the spark plug for cylinder 4. Look for signs of wear, fouling, or damage. If it looks bad, replace it. Next, swap the ignition coil (if your car uses coil-on-plug) or the spark plug wire with another cylinder and see if the misfire moves. That’s a surefire way to pinpoint a bad coil or wire. If the ignition parts check out, move on to the fuel injector. Listen for a clicking sound from the injector with a mechanic’s stethoscope, or use a noid light to check for electrical signal. If you suspect a fuel issue, you can swap the injector with another cylinder to see if the misfire follows. Don’t forget to inspect the wiring and connectors for any signs of corrosion or damage. If all that looks good, it’s time to check for vacuum leaks around the intake manifold, EGR, and PCV hoses near cylinder 4. Finally, if nothing else turns up, a compression test or leak-down test will help you spot mechanical problems like a leaking valve or worn piston rings. It’s better to have someone assist you with these tests, especially if you’re not familiar with engine internals.

Common Mistakes When Addressing the P0304 Code

One of the biggest mistakes I see is jumping straight to replacing parts without confirming the root cause. Swapping out spark plugs or coils without testing can waste time and money. Another common slip-up is overlooking simple things like a loose connector or a cracked vacuum hose. People also sometimes forget to check for mechanical issues-just because the ignition and fuel systems look fine doesn’t mean the engine internals are healthy. Skipping a compression test can lead to a lot of frustration down the road. Always work through the basics before assuming the worst.

Seriousness of a P0304 Trouble Code

This is not something to put off. A misfire in cylinder 4 can quickly lead to bigger problems-unburned fuel can damage your catalytic converter, and running rough can cause extra wear on other engine parts. Honestly, it’s dangerous to ignore this, especially if you notice loss of power or stalling. If left unchecked, you could end up with a failed catalytic converter, damaged pistons, or even a ruined engine. You don’t want this kind of trouble on the road, so act promptly.

Repair Tips for OBD Code P0304

Once you’ve found the culprit, the fix is usually straightforward. Most often, replacing a worn spark plug or a faulty ignition coil solves the problem. If it’s a bad fuel injector, cleaning or replacing it is the way to go. For wiring issues, repair or replace any damaged connectors or wires. If you discover a vacuum leak, fix or replace the affected hose or gasket. In cases where there’s a mechanical problem-like a leaking valve or worn piston rings-more involved engine repairs may be needed. Always clear the code and test drive the vehicle to make sure the misfire is gone before calling it done.

Conclusion

To sum it up, P0304 means your engine’s cylinder 4 is misfiring, and it’s not something to ignore. The risks just aren’t worth it-misfires can lead to expensive repairs if left unchecked. Start with the basics: check your spark plug, coil, and injector, and work your way up to more complex issues if needed. Acting quickly and following a step-by-step approach is the most reliable way to get your car running smoothly again and avoid bigger headaches down the line.

Video for "Error p0304" on YouTube

The following error codes are often searched for

P0339 P0340 P0341 P0344 P0345 P0351 P0352 P0353 P0354 P0355 P0356 P0365 P0400 P0401 P0402 P0403 P0404 P0405 P0406 P0410

Comments on the article