P0304

What P0304 means

Your car's computer watches how smoothly each cylinder fires by tracking small changes in crankshaft speed. Cylinder 4 is firing late, weak, or not at all, and the computer caught it enough times to set the code. This is a symptom code, not a parts code. It tells you where the problem is showing up, not what's causing it.

Is it safe to drive

Drive it as little as possible until it's looked at. A misfire dumps unburned fuel into the exhaust, and that unburned fuel can overheat and destroy the catalytic converter, which is a much more expensive repair than whatever caused the misfire in the first place. If the check engine light is flashing rather than steady, that's the car telling you to stop driving it now, not later today. A flashing light means the misfire is severe enough to be doing damage in real time. You may also feel the engine shake or hesitate under load, especially at idle or when accelerating.

What commonly causes it

Most often it's a worn spark plug or a failing ignition coil on that cylinder. Some engines use one coil per cylinder, others use a coil pack that fires more than one, so the fix and the cost depend on which setup your engine has. Beyond ignition parts, a leaking fuel injector, a vacuum leak right at that cylinder's intake, or low compression from a valve or piston problem can all cause the exact same code. Low compression is the one nobody wants to hear, because it means the engine itself has an issue, not just a part around it. Carbon buildup on intake valves is also common on direct-injected engines. The code alone doesn't distinguish between any of these. That's what the diagnosis is for.

What diagnosis involves and what it tends to cost

A proper look starts with pulling the code details and checking spark, then usually swapping the plug and coil to a known-good cylinder to see if the misfire follows the part or stays put. If it stays put, the next steps are a fuel injector check and a compression or leak-down test on that cylinder. That last step matters, because it's the difference between a $60 part and a much bigger repair. We won't quote compression work as a coil swap and hope it fixes itself.

Rough numbers only, since it varies by engine and what's actually wrong: diagnosis runs somewhere in the neighborhood of one to two hours of labor. A plug and coil replacement on a single cylinder is often a few hundred dollars in parts and labor combined. If it turns out to be a fuel injector or a compression issue, the cost goes up from there, and we'll walk you through what we found before doing anything.